A few things you could do:
-Tell your mother how you feel about the actions of your stepfather and why you think that changes are important.
-I think you already did this, but show her all the evidence you showed us. The picture of the room, the text you wrote, everything.
-This point might be a risk, so decide yourself whether you will do it, but remind your mother that she is defending violence.
-If she stays one sided: Call cps. I know your father told you that he has evidence against you, but I can’t believe that his evidence is stronger than yours.
-If all the points don’t work, escape without your phone and go to an irl friend that lives close to you.
If your parents find out that you posted the evidence and delete the posts, I will post a screen recording I made of the posts.