Yes they should be allowed to go. My wife is a grammar school maths teacher and they let all pupils use the bathroom if they need to. Its usually once during the course of a lesson the school permits them to leave the class unless they have a medical exemption or are unwell.
Many (id hope all schools) allow the pupils to drink water during class to keep them hydrated so chances are they will need to spend a penny at some point. When i was at school it was breaktimes only and there were a few instances i remember, especially in year 7 (11 year olds), of a few kids who needed the loo and being denied then wetting themselves in class. That sort of humiliation stays with a child for some time.
Being able to use the toilet is a basic human right, plus trying to hold it in can lead to bladder problems and urinary infections.
I think it's to early to start a fitting. you first should build a swing that works for you. after you are able to make desent contact on a regular basis it's time for the next step. so to start with standard should be fine
I can't give advice on this one, but I found myself that loosening my grip pressure (along with a package of other changes around softening my hands during transition) was the biggest change for getting my shots naturally straighter. It wasn't just a matter of deciding to grip looser - I re-gripped to midsize on advice from my pro to give me the confidence to do that.
I agree that it's probably not discussed that much on Youtube, but an instructor will definitely care about this, so that's a good route if you want to find out how its affecting your game.