As has been stated, and I hope you realize by now, you have nothing to worry about as far as court is concerned. She can not sue, has no legal precedent to sue, and you would not even have to hire a lawyer most likely even if she tried.
Everything prior to your GF turning 18, is off limits. Food, gas, clothes, etc. are required by law to be paid for by the parent/guardian. (this includes private school tuition fees).
College-if your gf agreed to pay for college, and they agreed on a 0% interest rate, that is technically an established verbal contract and you could be held liable for it. However given the cost of college, this would technically require a written contract. I assume there isn't one, and thus you are not legally bound to pay.
Rent-no. no, no, no, no. If you agreed to pay $50 a month, then that's the deal. If you agreed not to pay rent, that's the deal. Once again this establishes a verbal contract which is legally binding. Her increasing the rent is a violation of that contract if you do not agree to do so. If she is threatening to collect on fees associated with rent, she has no grounds to do so. Increasing the rent amount on a weekly basis without your agreement to that sort of rent schedule is illegal. (this means you could theoretically refuse to pay anything more than $50 a month, and she would be forced to try and evict you, which would prove harder than she might think).
Counter-suing: You could easily counter sue. This case would in all likelihood be handled in small claims court. Where you could counter sue for her actions on the grounds of emotional and financial distress. Not too mention her actions are technically a form of extortion, for which you could threaten to bring charges against her (this is not blackmail, it is only blackmail if you demand something beyond her ceasing her illegal activities).
My advice: depends on how you want to play it. I would just sever ties and go out on my own. If she threatens you with suit simply relay to her that you are aware she has no case, and if she did sue you would be forced to counter sue and highlight her extortionist activities which could result in her being charged with a felony. If you want to go loco, build your case, record her shady activities, record her threatening you with lawsuits, acquire evidence of her actions, and file charges.