@Phasmophobia said:

1. Why exactly was she turning into different creatures, including what appeared to be a werewolf and an octopus monster? Huh?

Her only explanation is that she is some sort of mutant. She herself doesn't understand why, but became an evolutionary biologist to learn how to live with it.

1. This woman has been alive for over 2,000 years. She has never been in love......until she meets this American dude and spends a couple days with him?

That was the point of the movie. For 2000 years she pushed the men away once she was pregnant like she tried to do to Evan. However, he didn't give up and eventually stayed with her to the end of her 20 yr cycle. Because of that, she fell in love with him and became mortal during her change.