The good news is you don't need anything from the PAH to create a brand new account.
The bad news is you can't port your number without the PAH releasing your number.
If you're financing a device it's THEIR responsibility to continue paying on it, but it's YOUR responsibility to return it to the PAH if they request it (the grey area is if it's paid off, they still have "proof" they paid for it unless you have further proof you paid for it). If it's being financed, I do NOT suggest using it as your device on your new line. Additionally, I wouldn't suggest using it on your new account in the event they report it lost or stolen (this is the hard spot because T-Mobile considers the phone belonging to the PAH, regardless of other transactions).
Hey there. I'm sorry to read about this disagreement -- it's definitely not ideal, and while family plans are a great option for most people in most situations, when things get tough they are tough.
smplyunprdctble is correct that ultimately the reason for the account restrictions that are designed in favor of the PAH/Billing Responsible Party are to protect the financial interests of the account holder. We've tightened security around porting out numbers especially with recent port-out fraud schemes in the industry. At this point, you'd need to work with the PAH directly in order to negotiate a resolution here to port your number out and settle any financial agreements made between the two of you. I hope that the other person is open to resolving this!