True. The technical definition of a blunder on lichess and chess.com is that it puts you in a losing position. You can blunder further while in the losing position. And your opponent can blunder which puts them in the losing position, only to be reversed by your blunder.
A mistake is like a blunder but you're still winning, but it could be a blunder in a worse position. An inaccuracy is a bad move that doesn't cost you.
Not quite, as that would imply that each game has at most one blunder. It's quite possible for two players to blunder back and forth multiple times.