It sounds like your grandma has been lying. Your dad has shown you physical proof and you have seen your insurance card. If your insurance card is current take it and go to the doctor. That is a reason alone to have you taken out of the house (not proper care of scoliosis). Geez, what a crappy family. Honestly, I would take baby steps to know your dad because it sounds like he was actively kept away from you all these years. Maybe he's a jerk, maybe he's not, but giving it a try and having public meetings with him and talking to him may help you figure that out.
No you won't have to ask your dad for permission, he doesn't have custody. If your friend's parents have custody THEY are who you answer to. That would be like my daughter asking her uncle (my brother) for permission when he has no custody at all over her... hope it made more sense with the example :) I believe you are at the age of where you can say who gets you and have a say in it, in regards to will you have to ask HIM for the friend's parents to have guardianship over you. YOU will not have to ask him, if he fights though your friend's parents will have to be prepared to put up a case to keep you with them.
You need to tell the court what your grandma has been doing with the child support. From the physical proof, she has been getting an allowance for you every month over the years and chillin on it. That is just wrong! Tell them about the insurance you have and how she has not taken care of your scoliosis, that is just wrong! She is unfit just from those 2 reasons, among MANY others that you have talked about.
If you need to get a representative through the courts. You need someone who has the knowledge on your side or your friend's parents need to get a lawyer because ultimately they will be the ones battling against your family, not you directly.
Are you with your friend's parents now? What about your sister? Glad to hear your so what happened. I hope it works out for y'all. I'm so glad you came to us for help and started this process :) Hopefully your sister will get her act together and be properly taken care of over there. I'm SO glad that you have friends and their parents that are willing to fight for you so you can be in a safe home. So so so glad to hear your staying at their house. Will you stay there until the custody hearing? Have y'all had that set up yet? Your grandma sounds very manipulative, so take anything she threatens you with with a grain of salt and write it all down in a log book for court to use against her... some cell phones can record phone calls too.