Don't do as I do, do as I say.
Tell them you don't get tipped for doing your job so you ought not be subjected to being made to feel guilty for not tipping others. If they are not paid enough then they should negotiate with the skinflint employer for a living wage, or find another job where they're not abused so.
1. accepted norm is whatever you feel right.
2. reasonable amount iw whatever you feel you should and can afford.
3. AFAIK you can ask for a service charge to be removed. Not sure if there has to be a reason though. Serve them right if you have the courage to, overheads for things like service should be covered in the price of the meal.