Honestly exactly what went on with your account is probably not going to be determined (but it could well have been hacked and that does seem to be the most likely thing).

The person now in control of the account has basically admitted that they did not do any due diligence about said account and have realised that they have screwed up (although they are yet to admit it entirely).

Unfortunately if you don't have a way to prove prior ownership of the account then there is really no solution here so perhaps you'll need to accept your new forum name (or create another one).

It is understandable to be upset over losing a name but also you need to realise that your password must not have been very good (otherwise your account would not have been hacked).