I think that self voting on the root post is an acceptable practice. I would compare it to a public official stating that they appove a message. However like you stated in your post:

From this perspective, maybe a root post and a comment are actually different. Maybe we should continue to vote up own root posts but keep the self-voting on comments to a minimum?

As a minnow with a very small following I am very selective on the posts that i upvote and even more so with comments. I try to only upvote comments that bring substance to the "conversation" so to speak. It is for this reason that I do not think that self voting on generic comments is a responsible practice. I did enjoy reading your thoughts and must say that I agree with most of the points you made throughout. Fair opportunity does not equal fair outcome. Just my 0.002 sbd lol