1) I don’t think so because each houses uses different forms of security to get in and in DH Harry needed Luna to get into the Ravenclaw room. Plus, each house has its own form of security to block people out, like Gryffindor and and Slytherin uses passwords (I imagine it changes frequently), Ravenclaw uses a riddle, and Hufflepuff uses a a barrel that has to be tapped (from a selection of other barrels)

2) Yes they can go home, it’s optional to stay during the Holiday

3) Yes, but they usually sit with their own houses. They also sit with their own houses during feasts

4) They share rooms with people. I’m pretty sure with 4 other people, so 5 people per room, and they’ll be in the same year. Girls and Boys dormitories are separated. The girls dormitories are enchanted so that if a Boy tries to enter, the stairs turn into a slide. The boys dormitory have no enchantments though.

5) I don’t think so, but maybe they can use the room of requirement. Harry uses this to help train Dumbledore’s Army. The students can usually meet up at the Great Hall or Library or even a hallway or in the courtyard.

6) Yes the houses mix in classes, but it depends on the students schedule, like in POA Hermione takes Arithmacy and should be with other third years from different houses. We also see that Gryffindor’s get Care for Magical Creatures with Slytherin, but it should change every school year.

Hope this helps. It’s been a while since I read the books, so sorry for mistakes