It is always sad when you cant see your mom or dad whenever you want. And it is difficult to share them with anyone, especially when they are not "your family" I think sometimes we tend to feel insecure at those times. But maybe not!
It is unfortunate that your mother speaks so poorly about your dad. That behavior will probably lower your opinion of her instead of the other way around. You are upset because you dont think your dad cares enough to pay for certain things for you. How do you know that the explanations your mom gives you are right/. If you really want to understand or know why your dad did something, you should ask him yourself. He may have better answers then your mom gave you.
I dont know about your step mom cause I am not there. But I do know about the hidden meanness! Or I think I do anyway. Actually I thought it was always a fake niceness!!! LOL LOL.

I would not compromise who you are but I would stay on your step moms good side. That would make your dads life more peaceful. And I am sure that would mean more time with him.
I am sorry you are feeling caught in the middle of your mom and dad. I think I would ask your mom not to tell you about their problems!
I hope your situation gets better soon.