Not renting a party room from them... but can I still bring in my own cake for someone's birthday?

Yes, if you bring an ice cream cake they will even put it in their freezer for you!

Yes you can I seen the same

Yes you can I seen others do that on the seating areas

Yes you can. You can bring decorations also. They have large joined tables toward the middle of the rooms.

Yes

Yes you can totally do that if you don't want their cake