MollyB, I am sorry that I didn't see this until now. I am so very sorry that your granddaughter is behaving in such a hurtful, ungrateful and terrible way. NONE OF THIS IS YOUR FAULT!!!! Please let me repeat this - Not one single bit of this is your fault.
You gave this girl everything. Literally handed her a gift that so many people would be incredibly grateful for. She not only threw your gift back in your face, she stomped on you as she did it. She is an ungrateful, spoiled selfish brat. I strongly suspect that she has some sort of personality disorder or character defect. Some people are just mean and wrong and won't see the error of their ways until they have burned every single bridge and are living under an overpass. It may come to that for her, and you must learn to be strong enough to let her deal with whatever the consequences of her actions are.
You continuing to pay her way will accomplish NOTHING but the loss of your self esteem and it will increase her belief that it is perfectly fine to treat people who are loving and generous and kind in this abominable way. Yes, her behavior is abominable. I actually cannot find enough horrible adjectives to describe how she is treating you.
What she is doing is gaslighting you. She is trying to tell you that something happened that didn't. She got caught doing something she shouldn't, she got caught in her own web of lies and she went on the attack. I swear there must be some sort of manual out there for twisted difficult children because this is pretty much a textbook attack. She is trying to convince you that things that didn't happen, did happen, and that things that did happen, didn't happen. It is ridiculous and rather like the 3 or 4 year old who is standing there with icing smeared all over his face trying to tell you that he didn't eat the cupcake, what cupcake, do we have cupcakes? When did we get cupcakes? Only with an adult it is calculated, nasty and ugly. It is also stupid. Just because you say something 50 or 100 times doesn't make it true. Sure, you may convince people who were not there and who don't listen critically to what you say that you are telling the truth, but that doesn't make it the truth.
Given the amount of money she is burning through, I suspect drugs are involved to some extent. Or maybe alcohol, but likely drugs. I would not give her a single dime. I can see maybe the phone if that gives you comfort. But if she uses that phone to abuse you, cut it off. Do NOT tolerate abuse or disrespect from her - you do NOT deserve it.
As far as her being in debt because of you, that is 100% bovine excrement. She is in debt because of her foolish ways. SHE signed those documents. You did not hold a gun to her head to make her sign. You did not choose the school, make her go and spend all that money, or in any other way drive her into debt. You gave her everything and asked for an incredibly small thing in return. You asked her to get an education. How DARE you! What a HORRIBLE thing to want! You wanted her to better herself! What an outrageous and awful request!!!!!! NOT!!! Her accusations are simply ridiculous. She is blowing hot air because she isn't getting what she wants.
It is time for her to grow up. You need to help her do this. The absolute BEST thing you can do for her right now is to NOT contribute to her toxic ways by supporting her. She needs a clear message that her behavior is unacceptable and until she changes and shows true remorse, she is on her own. She can deal with the debt, the boyfriend and whatever problems she has on her own. The rest of the family is right about letting her stand on her own two feet to face the consequences of her choices. Just because you CAN pay her debts does NOT mean you should. In this case, you should not. It won't help her and will tell her that she can continue in this way and continue to harm people with impunity.
I am sorry it is so painful. Posting here, to people who understand, will help. I also suggest you read the article on Detachment. You may find wisdom in the following books, "Codependent No More" by Beattie, and Boundaries by Cloud and Townsend. Both are excellent and are fairly easy to find. Given that you mention that granddaughter has had problems with drugs in the past, it would be a good thing to go to Alanon or Narcanon Family Meetings. Addiction is a disease that makes the entire family sick and the entire family benefits from treatment, not just the addict. I am the adult grandchild of an alcoholic. I didn't know it until I was an adult and I was flabbergasted. I was more shocked when I realized how much those alcoholic behaviors of my much loved grandparent impacted MY behaviors. I never even saw my grandparent drink. So your granddaughter's problems and use of substances DOES impact you and it is likely that Alanon or Narcanon will help if you let it.
(((((hugs))))) I am sorry that your granddaughter is so ungrateful and unloving. If I could have even one day with any of my grandparents, I would treasure it and give up any amount of money. Your granddaughter is a fool to treat you like this. An absolute fool to turn her back on so much love.