While generally I prefer they become friends, I think the LNdid a good job of making Alicia's betrayal make sense; she's just crashing out because Yumiella has (accidentally) traumatized her and is the most dangerous person alive, while she hasn't gone through any of the growth the game's story...
I believe this is following the anime story. The LN author has said he likes the different direction that the anime takes the heroine in, and anime is afaik more popular than LNs.
It seems like they had their own recovery items, but it's prob better to get the ones that the dungeon provides, so you can save your personal stock. The taste is also a nice bonus
I'm pretty sure he is real, as the others have acknowledged a "real" version of him when she's having delusions, she just has schizo hallucinations of him when he isn't present