What can I do, being on a Tenure-track position and noticing that the promised start-up package, course release and ... were just false promises?
My strong advice is that you take an offer from a better school and just do what is needed to get through the year as best you can. If the place is as bad as this then you, with little institutional power behind you, aren't going to make a difference. You will be stomped like a bug. I was once in a bad situation (perhaps not this bad) and went for the door by the shortest route possible, simply saying goodby to the insanity. It worked out fine. As in your case, I wasn't going to get anything from any other action.
In particular don't say things that might come back to negatively affect your own career. Suing them might be an option, but it requires a hard talk with a lawyer first and threatening it isn't going to get you a positive outcome.
I realize this is hard advice to take, but you have a good option and you don't want to do things that antagonize people and that will cause blow-back on yourself.
You are fortunate to have a good option. Take it. Others aren't so lucky.