If Eric is wanting the house then for sure he won't stay for the whole game.
In war route you definitely will won't to kick him out.
On friend route he sooner or later will kick MC out leading to game over.
The only sane way to keep Eric in the game for long haul is to let Max have some kind of influence on the family as the only son. Don't make Eric super evil just make him lustful with no heavy influence on the sisters atleast.
Make Alice & Lisa lean on Max as he has been helpful to both girls always. Make him lovable and not a douch. This way eric will not become all powerful and a healthy competition can be held between both men for the women of the house.
As for the Adult movies, Kira can convince Max & Alice to join her as they both need money. Lisa can also join them later in the movies. This aspect of the game can be kept a secret from Ann & Eric which will create a sense of mystery & tension in the game which is good too.
Where there's will, there's a way. just saying that there is no shortage of ideas if you really want to do something.