Hey everyone, im looking for some advice with an upcoming session. My plan is to have them enter a dream like demi-plane while trying to get through a cult occupied town while on their way to the main quest. The things I want to ask are as follows

1) the main way that the town enters this dream plain is through dream crystals in the middle of town, and these crystals hold a creature inside it. Would it be a good idea to have a mind flayer inside it thats actually good and just wants to go home?

2)what creatures in habit this realm? Any that stand out? Or do I just use dream like creatures

3)would it be to over powering to have the group interact with the dream scape?

4)would nightmares be a good way to prevent the party from staying to long?

I appreciate any and all advice on this.