I think the Alien was something the movie could have done without, just focus on the Neomorphs instead. Either that or simply just keep it to one Alien instead of two, as it doesn't feel so persistent and stalker-like when it is a completely different individual on the ship instead of the same Alien chasing them fom the citadel.
Faris tripping never bothered me too much, her hypocriticism did though. She locks Karine in and refuses to let her out due to contamination, yet Faris has Ledward's blood all over her face, making her just as much if not even more a contamination threat than Karine was.
I personally don't agree with the sexual frustration thing as you would need to have sex organs and feeling to be able to be sexually frustrated in the first place. That being said, who knows what goes on in a synthetic brain.