It sounds like you need a place to vent and that’s reasonable. You are understandably very upset. This is a place where you can get it all out without impacting your son. So vent here.
In case you’re not just venting, though, and would like a little feedback from an unbiased perspective… get all this out of your system before talking to your son’s school administration, much less before you discuss any of this with potential colleges. Since you’re not going to get legal help with this, an adversarial approach will be counterproductive. Calm and reasoned may get more cooperation from the school - which you and your son will benefit from. The fact that they’re starting to shut down communication with you is a bad sign. You want them working with you, not circling the wagons to ward off the crazy lady. And it is incredibly tough and frustrating to be restrained and measured when you’re worried about your child, but think of it as just another tough thing you do to help your son.
As for colleges, hopefully this will all be a nonissue by then. If you can work this out with the school and your son can get back on track, this will just be a speedbump that can even be fodder for a good admissions essay. Again - vent here - do not discuss any of this detail you’re posting here with a college. If worse comes to worst, your counselor can mention to the colleges that your son had a medical issue that impacted his studies in 10th grade but that he persevered and continued to do well in future grades.