Sorry but this is your man's duty, not yours at all
the bible says a man is to leave his mother and father and cleave to his wife
even if you're not christian this is just common sense, if he much as plays "mr. Stuck in the middle" for one second, he's just enjoying the tug of war, he's not fulfilling his duty to you, and you need to give him an ultimatum...If he wants you it's 100%, not anything less
he needs to tell his mom that he will not listen to wife bashing, and if she persists he will just leave...In short, he's got to be a man for the first time in his life and stand up to mummy and remind her that he's going to be leading his own family soon, and her days are done, she can go take a cruise and shut up...
This will be the dynamic of your relationship for as long as she lives if you do not get this ironed out now, absolutely guaranteed, and you will be a shriveled hapless mess with your in-law gloating that she has maintained her power...But it's not her fault, it's your hubby-to-be...He'd better grow up or you'd better leave or just wait until he does grow up before you ever marry him...