There is not much mentioned in the Vedas about Christianity or Islam. The Bhaviṣya Purāṇa is a book of predictions, but there are several versions, and I don't know which might be authentic, so I haven't paid much attention to it.

I was taught in disciplic succession that originally there was one Veda that was passed down orally, beginning with Kṛṣṇa, the Supreme Personality of Godhead, to Brahmā, the first soul in universe, who taught it to his offspring, to the present day about 155 trillion years later. The Veda was divided into four by Vyāsadeva, who added a fifth Veda of history books to explain it.

The main branches of the Vedas are logic (Nyāya), computational (Sāṅkhya), atomism (Vaiśeṣika), causality (Pūrva-mīmāṁsā), applied (Yoga), and the conclusion (Vedānta). Several original developers of modern quantum theory praised what little they heard about Vedānta for enabling them to somewhat comprehend what their math was telling them.

Sāṅkhya describes reality without anything resembling gravitational theory. Gravity is a theory of motion, and Sāṅkhya explains the motion of both living and nonliving bodies with one force called prāṇa, which acts in five different ways. https://www.ashishdalela.com/2017/01/28/what-is-prana/

My understanding of the nature of human beings based on the Vedas is that nature is three qualities, sattva, rajas, and tamas, which are often translated as goodness, passion, and ignorance. Life is fundamental, and these three qualities are produced from interactions between the three primary aspects of life, sac-cid-ānanda, meaning existence-knowledge-bliss.

Everything emanates from Kṛṣṇa, the independent whole and supreme enjoyer, and material nature is experienced by those of us who envy Kṛṣṇa. Forgetfulness of Kṛṣṇa overcomes the envious living beings who then experience the opposite of our true nature, which is repeated birth and death in ignorance and frustration. Remembering Kṛṣṇa activates our true nature and is accomplished in the present age most effectively by chanting Kṛṣṇa's names without stopping. There is no limit to the bliss that can be produced from Kṛṣṇa's names.

You are correct that the Vedas cannot be comprehended only by reading. Kṛṣṇa says in Śrīmad-Bhāgavatam that the Vedas can only be understood in conjunction with careful devotional service. One external reason for this is the formal Sanskrit can only be perfectly spoken and heard, not accurately written. Therefore that portion of the Vedas is called śruti, which means "heard." A vernacular Sanskrit is used in the fifth Veda, which includes Śrīmad-Bhāgavatam, the Mahābhārata, Bhagavad-gītā, the Rāmāyaṇa, etc.

The Vedas are meant to be taken literally, unless a specific context indicates otherwise. Correct understanding of the Vedas is advised to be received through disciplic succession. There are four ancient Vaiṣṇava traditions that offer complimentary descriptions of the ultimate reality as Viṣṇu or more specifically Kṛṣṇa, and a relatively modern tradition of Śiva worshippers who promote Advaita Vedānta based on Śaṅkarācārya's early teachings instead of the conclusion given by Vyāsadeva.

What we hear depends on our goals, but hearing śruti with correct pronunciation and understanding of grammar is technically unambiguous. The Vedas explain how best to obtain their knowledge, which includes paramparā, which means learning from someone who knows it. A quarter century ago I searched for the best knowledge and stopped searching when I found Śrīla Prabhupāda's books, which is almost entirely what populates the Vedabase that I linked. It gives original Sanskrit, word for word translation, a translation of the whole verse, and an explanation.

Sanskrit grammar has a tree shaped hierarchical construction that can program computers, and I recently noticed there is a lot of in depth modern writing about Sanskrit computational linguistics but have had time to read enough of it to represent it. I have never had the opportunity to learn more then a little Sanskrit myself, but my understanding is anyone who knows Sanskrit grammar perfectly would also know the meanings of all potential words, as explained here: http://uttishthabharata.wordpress.com/2011/04/20/sanskrit

There is no vacuum, and the Vedas can be understood from within if a person finds how to access them.

Kṛṣṇa said,
SB 11.21.37: As the unlimited, unchanging and omnipotent Personality of Godhead dwelling within all living beings, I personally establish the Vedic sound vibration in the form of oṁkāra within all living entities. It is thus perceived subtly, just like a single strand of fiber on a lotus stalk.

My understanding from the Vedas is that in practice the ultimate reality can only be learned from someone who knows it. Perfect knowledge can't be manufactured from independent research; it always exists but is deeply hidden until awakening by someone who has it manifested. I'm one of many who believe Śrīla Prabhupāda empowered his books with such Divine potency. There are two links here to Ashish Dalela's site, who explains the Vedas' theory of everything in a way avoids the internal conflicts in modern science.

Having found one theory of everything that I am convinced is consistent and complete, I didn't think another would be possible with the same or fewer axioms. If my understanding of Gödel's Incompleteness Theorems is correct, any theory that uses mathematics can't qualify, because it isn't possible to make a complete and consistent set of axioms using numbers.