I was never allowed to put a child seat anywhere but in the window seat.

Bathroom breaks can be tough. Never had a FA offer to help and I never asked. Sometimes, you will have a sympathetic seat mate. Other than that, you will have to take baby with you. It's doable; pick your traveling clothes with care and don't wear something difficult to zip up and tuck in. I wouldn't leave my child alone in the seat. Of course, that's the moment they will wake up and start screaming when you aren't there.