I got to the point where I have permission to enter her room at night, even if the door is closed, but it still says I need permission when I try. What do?

I haven't had this issue reported before; have you tried asking "Is it all right if I check in on you at nighttime?" under the behavior menu, again? It should unlock all of the options you currently have available based on your stats. Entering requires Permissivity 3.

My impression is that getting permission to enter when the door is closed only applies if she's sleeping, not if she's masturbating or hooking up with someone else. Maybe content for the latter two scenarios has been planned but not written yet?

I've doublechecked, and visiting in night-time is still working. You do need special permission for entering while Terry is asleep, which happens at Permissivity 4; asking, "Is it all right if I check in on you at nighttime?" will unlock any permissions that are currently available for you. I'll clarify things so it's clear that entering when Terry is asleep is a special case.