1) Would you like to be able to Accept multiple answers?
No. It happens too rarely that two answers solve the problem with an equivalent quality. And then the OP can vote and write a comment to mention this. The under-compensation of the not-accepted answer could be compensated by a much more frequent voting of answers.
2) Should Accepting an answer automatically upvote it by default?
No, the acceptance is a enough.
3) Have you ever wished that you could upvote a comment, and not just answers?
Yes, then I ask the author to post this as an answer, such that I can vote for it. This is successful usually.
4) Question - is there any way to change which answer you've accepted?
Yes, this would be an important improvement. In some cases I have "un-accepted" my answer by deleting and re-posting it, but this works only if there are no comments, which would be deleted also. I think the OP and the editors/admin should get the same power for un-accpeting as for accepting.