Most of those comments are coming from people like you that have no idea what they are doing and no experience. RedGard is a waterproofing "film" once dried. Why would you apply it then try to peel it off? That's just plain a stupid thing to do and proves nothing. It is a film, of course it will peel off. Why would anyone try to peel it off?
You should get going with your project and stop believing every little tidbit of rumor you come across. My God, this is beginning to read like a "True Detective Magazine". All fiction.
A primer-coat is a good idea and sanctioned by Custom Building Products but not necessary. It is bunk coming from someone that thinks they have a better idea.
You must make a decision as to whether you are going to listen to experienced installers or non-experienced DIY'ers on a forum. How many forums are you going to for advice on this single subject? The more forums the more confusion.
Do what you want.