All that really matters is it is expressly forbidden, per CX crew that have opined on other threads
Reason enough, but...
stealing time in a lav you're not entitled to use
So she'd use the J lav. Problem solved.
you're stealing space and atmosphere from the cabin
...by sitting in the visitor seat that's already part of his allocated space?
on this most vile of topics.
Please tell me that was a joke. The hyperbole is only hurting your argument.