Sorry, you havng such an unpleasant time with this.
Just a thought but if your grandmother (her mother) is around and you're close to her or perhaps an aunt, maybe speak to one of them and they'll talk to her and tell her it is out of order (especially if shes close to them- the impact of that would be quite severe)
A school guidance teacher might be good to talk to if you are at school and you trust him or her but if you are still legally considered a minor, your state might have child protection laws which would require that they inform social services or whoever deals with this sort of thing.
If it comes to it tell your dad and regardless of what he may or may not be doing from what you've said he's atleast tried to protect you from what he may or may not be doing and if your mother felt the need to have an affair she should have kept it away from her kids rather than drag them into it. If shes complaining about you being mean, then perhaps thats you need to be difficult though it is as it doesn't look likely that shell change her ways and I'd say bringing some random guy home who you have no idea of who he is, is exceptionally mean. She should have to remember she is the mother not you and if she is doing this to get back at your dad, then it is bizarre that shes making you suffer.
Hope this is resolved soon.