If a question-writer downvotes an answer, then the answer is modified after being remodified because it was missing information, can the reputation come back if the question-writer accepts the answer? Can a question-writer accept an answer he downvoted? If so, does it undo the downvote? Does it cost the question-writer reputation to go back on his choice?
Also, how is it that I can still chat if I now have 19 rep? I just lost 2 rep for being downvoted (which is why I asked that last question in this chat)
@user2778427 You have accounts on several other sites. Chat privileges are determined by the sum of your reputation on each site (it's a bit more complicated than that, will explain in a bit). If you have at least 20 reputation total, then you're allowed to chat.
I just saw that apparently Super User has its own chat rooms or am I mistaken? While Stack Exchange itself has its own chat rooms (including this). Because I have 19 in Super User yet it shows I am about to earn access to Super User's chatrooms. Is this correct?
Technically, your chat reputation should only include sites with which the server is associated. This is chat.stackexchange.com; your total reputation on these sites is 21. For some reason, it's currently showing as 22 (possibly due to caching).
@user2778427 That's tracked at the site level; however, the chat server grants privileges based on your total rep for all sites associated with that server.
(seems to be a bit glitched, though; went through my own account and the numbers don't quite add up, at least not within rounding tolerances - could be a bug with the way chat tracks reputation)
Stack Overflow and Meta Stack Exchange each have their own chat; the rest of the network (Stack Overflow in non-English languages, Super User, Server Fault, Stack Exchange 2.0) has a combined chat site (Stack Exchange chat). Privileges apply based on your total reputation on the associated site(s...
@user2778427, regarding your question on voting, unless someone indicates in a comment that they are voting up or down, there's no way to know who did the downvote. It's less likely to be the question author, because authors usually don't downvote honest attempts to be helpful. Each vote or acceptance is independent as far as effect on your rep. A person can retract a vote within a certain time window, and all its rep effects are reversed.
I don't think there's a time limit for the OP to unaccept or change an acceptance to a different answer. Unaccepting reverses all associated rep effects.