OK to respond to your last point "obsolete comments" question - any time a comment is no longer needed say for example: the post was edited, a bounty is long gone, other comments were deleted, etc. Flag the comment as obsolete.
If there is an answer, especially a good and UVed one, but the question has since been edited and now it no longer answers the new question, you have a few options depending on the circumstances.
Say the question was simple, and got a answer; however now the OP (a new SE user) comes back and has a new question, so he edits his question. In that case do nothing to the answer, the OP should be made aware that this is not how SE works, and you could flag the question (please use a custom message saying in a few words what happened).
Now there is a question, a bit unclear, it gets answered. The OP comes back and edits, maybe adds a blend file, and now that answer definitely does not answer the question. In this case DV the answer. It would be nice if you also left a comment as to why you are DVing the answer.
The third scenario - the question doesn't really matter, its there gets an answer then the OP edits and might even answer himself. Before it got edited, a different user gave a really good answer, except in this particular case it had nothing to do with the problem the OP was having. As later discovered, it was because the OP had spilled grape juice on his computer, which killed only part of his hard drive. In this case I would not DV the answer, I might cast a close vote on the question as "too localized," but I would not do anything to the answer (you could UV it if it helps you.)
(did this come up from this answer?)
A possible example of both points is in my answer here, and the comments below.