Oh Stan, Stan, Stan. Where to begin in explaining things? Well, let's start here:

First, you compare the new versions to the KJV as if the KJV were the gold standard of translations. It's not. Not even close. I was written at a time before we had discovered many of the most ancient and reliable source texts. The preface to the New King James Version makes this quite clear (though it's not in dispute):

Since the 1880’s most contemporary translations of the New Testament have relied upon a relatively few manuscripts discovered chiefly in the late nineteenth and early twentieth centuries. Such translations depend primarily on two manuscripts, Codex Vaticanus and Codex Sinaiticus, because of their greater age. The Greek text obtained by using these sources and the related papyri (our most ancient manuscripts) is known as the Alexandrian Text.

After comparing some of these various ancient source texts to the Alexandrian Text discussed above and noting some discrepancies in the ancient documents, the Preface to the New King James Version predictably attempts to minimize the significance of these differences:

[I]t is most important to emphasize that fully eighty-five percent of the New Testament text is the same in the Textus Receptus, the Alexandrian Text, and the Majority Text.

Again, virtually none of this information was available to those who translated the KJV. So, you shouldn't be comparing new translations to the rather naive and ignorant KVJ but instead to the most ancient and authoritative texts--the Textus Receptus, the Alexandrian Text and the Majority Text.

And, when you do that, you find that many of the doctrinal "changes" to the more recent translations that concern you shouldn't. The KJV depends heavily upon Tyndale's original English translation, and Tyndale is well known to have taken certain liberties that subsequent translations have merely corrected:

It was Tyndale who established …that the Bible should not be in the language of scholars but in the spoken language of the people. [H]e coined such words as “Passover,” “scapegoat,” “mercy seat,” and “long-suffering.”

Many expressions of Tyndale are also unforgettable, cherished by countless readers of the English Bible: “the kingdom of heaven is at hand” (Matt. 3:2); “the pinnacle of the temple” (Matt. 4:5); “the salt of the earth” (Matt. 5:13); “daily bread” (Matt. 6:11); “Consider the lilies of the field, how they grow” (Matt. 6:28); “meek and lowly in heart” (Matt 11:29); “shepherds abiding in the field” (Luke 2:8); “eat, drink, and be merry” (Luke 12:19); “fatted calf” (Luke 15:23); “only begotten son” (John 1:14, 18); “in my Father’s house are many mansions” (John 14:2); “in whom we live and move and have our being” (Acts 17:28); “God forbid” (Rom 3:4); “sounding brass” and “tinkling cymbal” (1 Cor. 13:1); “in the twinkling of an eye” (1 Cor. 15:52); “singing and making melody (Eph. 5:19); “office of a bishop” (1 Tim. 3:1); “the pleasures of sin for a season” (Heb. 11:25); “an advocate with the Father” (1 John 2:1); and “Behold, I stand at the door and knock” (Rev 3:20). (How We Got the Bible at 178-179)

Despite their familiarity, the above expressions do not appear in the original Greek, at least not in so many words:

[I]t is noteworthy that these expressions could have been translated differently from the Greek text, yet because Tyndale had such an ear for the English language, these [para]phrases live on. [parentheticals added]. (How We Got the Bible at 179)

Here are many more additional examples of why the KJV shouldn't be your benchmark for determining what is or is not canonical/doctrinal (some of these you mention in your original post as being concerning):

1. Later manuscripts of 1 John 5:7-8 contain a phrase that the earliest and best manuscripts do not. Specifically, later versions say “in heaven, the Father, the Word, and the Holy Spirit, and these three are one. And there are three that bear witness on earth, the Spirit…”. This phrase first appears in Miniscule 61, dating to the fifteenth or sixteenth century. The reader can see How We Got the Bible at 100 for an interesting discussion of how this verse came to be included in modern English Bibles, but to make a long story short, it was originally inserted by Literalists centuries after the fact to bolster the doctrine of the Trinity.

2. Later manuscripts of the Gospel of Mark contain an ending that does not appear in the earliest versions of the book. Specifically, Chapter 16:9-20 of Mark, which describes Jesus’ post-resurrection appearances to Mary Magdalene and others, do not appear in the earliest and bests texts, such as the Codex Sinaiticus and Codex Vaticanus. Likewise, they do not appear in the oldest known manuscripts of the Latin Vulgate or Old Syriac. The vocabulary used in these verses is highly suspect and generally does not fit with Mark’s style. Furthermore, many expressions found in them appear nowhere else in the entire New Testament.

Because all of our earliest versions of Mark simply end his account with the discovery of the empty tomb (Chapter 16, verse 8), we can be assured that the expanded ending placed in our modern Bible is almost certainly inauthentic. After all, consider which is more likely: (a) Early orthodox Christians (who argued vehemently that Christ physically resurrected “in the flesh” and who were the keepers of the Bible for centuries) inexplicably deleted Mark’s original account of Jesus’ post-resurrection appearances before compiling our most ancient texts and destroyed all older versions, thus accounting for its absence in our earliest manuscripts; or (2) these same orthodox Christians intentionally added references of Jesus’ post-resurrection to the end Mark to support their argument that he rose in the flesh and inadvertently failed to destroy all older versions. If we are to be fair, the second explanation is far more likely.

1. In Mark 1:1, many of the oldest manuscripts do not include Jesus’ title as “Son of God.” This “divine” title was added by the church to bolster its claim of Jesus’ divinity.

2. The oldest and best manuscripts mistakenly attribute the Old Testament quote in Mark 1:2 to Isaiah rather than to Malachi. Later scribes corrected this obvious error by altering the text to attribute the subsequent quotes simply to “the Prophets.”

3. Mark 11:26 does not appear at all in the most ancient texts.

4. Mark 15:28 does not appear in the most ancient texts.

5. Part of Luke 22:19 and all of Luke 22:20 do not appear in the most ancient manuscripts.

6. The oldest manuscripts do not contain John 8:1-11, which records the only gospel account of the woman caught in adultery, and Jesus’ famous statement to “let he who is without sin cast the first stone”. About a dozen of the ancient manuscripts (the “Family 13” miniscules dating to the ninth century) peculiarly place the story of the adulterous woman not in the gospel of John, but after Luke 21:38. However, based on the historical record, scholars are quite certain that this story was never part of John’s or Luke’s “original” account.

7. The oldest manuscripts do not contain Acts 9:37, in which Phillip apparently requires a eunuch to profess a certain “belief” that “Jesus Christ is the Son of God” prior to being baptized with water. The eunuch’s confession of faith appears for the first time in the Codex Laudianus, which is dated to the sixth century. This spurious addition was inserted long after Acts was written, likely in an effort to bolster the orthodox position that specific “beliefs” were required in order to receive baptism and be saved.

8. Matthew 23:14 is not found in the earliest manuscripts.

I can't emphasize enough that the most important of these differences between the oldest (and newest) manuscripts/translations and the KJV--that is, the additions that appear in later manuscripts like the KJV but which do not appear in earliest (or newest) versions-- are easily traced to on-going and well-known debates within the church on various theological issues. Perhaps not surprisingly, virtually all of these edits tend to bolster what we would today consider to be orthodox views (but which were highly contested then).

For example, the doctrine of the Trinity was not an explicitly stated theological concept when the earliest manuscripts were written, and it was the subject of much heated debate as late as the 4th century CE (and actually much, much beyond that, as any moder day Unitarian would tell you). That later manuscripts contain Trinitarian words or phrases which are not found in the earliest versions of those same manuscripts is compelling evidence of intentional revision by orthodox editors, not simply scribal error. Even Bible scholars frequently cited by Literalists have had to admit this fact:

I think that these words [concerning the Trinity in 1 John 5:7-8] are found in only about seven or eight [source] copies, all from the fifteenth or sixteenth century. I acknowledge that this is not part of what the author of 1 John was inspired to write. (Lee Strobel, The Case For Christ at 65, quoting Bruce M. Metzger, PhD)

The insertion of Jesus’ title as Son of God in later versions of Mark 1:1 provides a second example of a spurious and intentional addition. Jesus’ exact nature continued to be debated into the Fourth Century, with many Christians viewing him as having been purely human, others as purely God, and yet others as both. Those who viewed him as in some manner divine ultimately won the day. That his “divine” title as “Son of God” appears in later versions of Mark 1:1, but not in earlier ones, again reveals the hand of editors intent on bolstering their theological case, and perhaps on harmonizing the Gospels. (As a side note, "Son of God" was not considered a divine title at all to Jews, as evidenced by various Old Testament personalities who, like Jesus, were called such. However, Greek culture, thanks to various pagan religious myths, did consider the "sons of God" to be divine.)

Yet, another example of intentional revision occurs in Luke 2:33. Originally this verse said, “And His [i.e., Jesus’] father and mother marveled at what was said about Him”, suggesting that Jesus’ father was Joseph, which could not be if Jesus were born of a virgin. Consequently, Literalist scribes altered later versions of the manuscripts to read, “Joseph and his mother marveled at what was said about him”, thereby preventing anyone from interpreting Luke as suggesting that Joseph was Jesus’ father. The altered version is the one that is preserved for us today in King James Version of our Bibles, although the New American Standard and New International Versions are among those that properly render the phrase as it was originally.

Orthodox editors also made revisions to Luke 3:22 to counteract the claim by some early Christians that Jesus was not truly divine until he was “adopted” by God at his baptism. This theory, known as adoptionism, was popular among certain early Christian groups and, at least originally, had biblical support. For example, the oldest surviving source texts of Luke 3:22 quotes God as saying from the clouds at Jesus’ baptism, “You are my son, today I have begotten you”, which is a paraphrase of Psalm 2:7. However, orthodox scribes, who opposed the adoptionist theory, were naturally uncomfortable with a quote from God suggesting that Jesus was not begotten of God until the day of his baptism, especially when such language was different from that offered in Mark’s and Matthew’s accounts of Jesus’ baptism, and directly contradicted John’s account of Jesus’ pre-existent divinity!

So, was Jesus begotten of God before his birth, as suggested in the opening chapter of John, or only after his baptism, as taught by the authentic Luke, or only upon his resurrection as taught by Paul? All of these inconsistencies were too much for Literalist orthodox editors to bear. To prevent the faithful from being “misled”, Literalist scribes and translators simply altered Luke 3:22 to read, “You are my beloved Son; in You I am well pleased”, an apparent illusion to Isaiah 42. This alteration had the three-fold benefit of preventing an adoptionist interpretation, harmonizing Luke’s account of God’s words with the accounts of Mark and Matthew, and linking Jesus to Isaiah and his prophecies. Perhaps it is for these reasons that the altered version survives to this day in virtually every modern translation of the New Testament (though the authentic reading is sometimes footnoted).