How does “Reputation” work?
If I downvote accidentally, or downvote first and later realize that it's not worth downvoting and undo the downvote, do I get back the 1 reputation point lost because of downvoting?
Yes, you get the 1 reputation back. Undownvoting in all senses is literally a reversal of the original downvote. Meaning all reputation effects of the downvote are also reversed.
After 5 minutes your vote is locked. You can't change it unless the post got edited. Then you can change your vote.
But this should really really not just be because of that 1 rep point you lost (Besides you only lose one point for downvoting answers and not for questions).