"why does Kainen lose his immortality when it comes to love? "
- The reason why he loses his immortality is because the mangaka hints at the fact that belials who love will become weak and die. It's a bit cliche-like, as other stories with demons, vampires, darker creatures, etc who seem invincible will die/lose immortatlity/etc when they fall in love. Hope that helped....
" and why does Kainen say you are born immortal but Gatmiel says you must kill the parent to become immortal?"
- From what I could understnd from reading the series, I believe that when a belial is born, the mother dies in childbirth (?).
"and is Gatmiel half Belial?"
- Although not directly stated, I believe this is what the mangaka indicated.
"and why in the sequal to Masca does it have Rakin say he is no longer immortal?"
- I have not read this anywhere, so I don't know where you are getting this from....