I think it's hilarious that her mother seemed to approach it from the right direction, then after this it went so, so badly. But I guess that's what we were here for, not her mother making the correct decisions, but the bad ones
Okaa-san is smart, it would be a shame for her daughter to ask anyone advices of "how approach sexually your mom"
She should be there for any problems her sweety daughter must have
Lots of interior doors can have their locks overridden by coins or pens depending on whether they have a slot or hole to release the lock. Let's the doors be locked for privacy but they can be relatively easily opened in an emergency. In the states at least, I've tended to see more of the "hole" type.