A: Yes – the final scene shows them together, though Shinichi admits he’ll never be fully “human” again.
A: No. The anime adapts the entire manga. There’s a live-action movie (2 parts) and a stage play, but no sequel anime.
A: Similar themes: Devilman Crybaby (humanity vs. demons), Tokyo Ghoul (identity loss), Inuyashiki (elderly man vs. alien weapon), or Kiseijuu: Reversi (a short manga sequel – different characters).