To try and dance around major spoilers, the events in the final route mean none of this matters.I'm curious about this myself. Everybody else is talking about camping skills but what about STORY? In Alice route, her mom dies before you make it to the castle, but she's still there with ilias route. Whichever side you pick, the other girl will get two companions that they obviously otherwise don't have if you're with them and then swap. End of pure alice part 3, Ilias entrusts her recruited dog and slime to you but if I had been with ilias does that scene get changed? Removed entirely?
Very spoilerish answers to specific questions
Once the chaos route of part 3 starts you can save and recruit Alice's mother
As far as I understand, the companions Alice gets in her own route kind of disappear except for the major story characters like Tamamo. I think there is an implication that they are alternate versions of Ilias's but that might be too much speculation
Ilias's monster companions join you in the chaos route of part 3 as NPCs regardless of which side you originally picked. Towards the end of the game they become playable.