Well, you have the upper hand since you can recover the acc manually by emailing jagex. It's your choice man, honestly, I would give back like 85-90% for wasting my time...
no, you wouldn't. by that point you would have already given them information that would make it possible for them to threaten the security of your account.
You aren't required to give a refund. However, you should say if you accept refunds in your term of service or agreement before payment. It's up to you, as long as you didn't guarantee any refund.
No, they've probably already resold the account and are trying to screw you out of your end of the deal. This happens all the time. Tell them to fuck off