My son went through the exact same situation, even the grand mother thing. He got DF, contacted me (cause they couldn't stop him anymore), so now we have a good relationship even if it had been severed for 15 years.
Welcome
I got baptized at 18, and by 19, I was ready to get married. In Canada, some elders were recognized by the legal system to perform marriages, so of course, being a JW, I contacted one to get married. He didn't like me, thought I was a cocky little bastard (which I was....LOL) and refused to marry me. He thought we were too young (which was true), but anyway.....
When he told me he wasn't going to marry me...(two weeks before the ceremony) his conscience wouldn't allow it he claimed....... I didn't say anything and setup to get married by a Justice of the Peace officer. Whoa....when they found out that I was getting married outside their arrangement, on the arranged date (without a glitch), and without even discussing the matter with them, they scattered like cockroaches under light, it was truly funny. They hate it when you go get outside help, it brings reproach on the org. they claim A meeting was setup with four elders, plus the elder that didn't want to marry me, my wife, the mother in-law ....and there I was sitting with all the power and a big grin on my face as I watched the elder beg me to allow him to marry me. So yes we did have a nice theocratic marriage, with the loving elder performing it having to put up a front and hating every minute of it..........
So that said, you need to contact a lawyer, or at least threaten to contact one. Call the elder, tell him you will not be meeting with them until you find proper legal advice. If they wish to discuss, you require that your lawyer be present (and stick to it). If the threat (or bluff of it) is not enough to keep them off your back, well...you are a minor, and you can easily find legal advice, especially when it comes to family issues, emotional or physical abuse and harassment. A legal aid lawyer would certainly look into the matter and have a field day with these guys. Trust me, the elders won't even want to go there until you're 18.
Good luck, keep us posted.