You are well within your rights to use the handicapped stall. It’s an accommodation not a reservation.
Yes, just like those convenient parking spaces and seats in the tube
I'm a man, but I have always understood the dynamic to be completely different in (U.S.) women's restrooms. There, women chitchat with complete strangers. In that case, the current user would cheerfully respond, "Yes, I need a few more minutes" and that would be that. No embarrassment whatsoever.
Reminds me of the episode of IT Crowd where Roy pretends to be disabled.