No Samsung doesn't cover any warranty for cracked or damage screen. It cover the warranty for any glitch or any malfunction, but not for any broken parts.
The cost for the Samsung phone screen differs from model to model. If we know the exact model for your phone then it will be easy for us to give you a price.
Yes you can get Original screen, brand new or refurbished. No after market screen can set or work properly on Samsung Phone.
Black spot is know as screen bleeding, yes you can use the phone in that condition, but as time goes the bleeding or black sport generally grows. Once it come to that lever that your can't use, than you need to replaced the screen or sure.
It can be a issue with the screen or display chip which is on main board.
May be the Phone is in a different mode or some time the main board will get dead.
Yes we do change glass on most of the Samsung Phones, but it depend how bad the screen got shattered and which model. And the prices differ as per the model.
Yes sure, Samsung use HARD OLED till S7, S7 edge, NOTE 5 from S8 and NOTE 8 they started using SOFT OLED.