invalidating your baptism is a hard sell and difficult to prove. Even if you can prove that you were not technically valid for baptism at the time, I've seen the borg argue that your living as one baptised afterward proves that it was valid. I've also seen them pull out the "it was directed by Holy spirit so it must have been valid" card as well.They will talk from both sides of their... just to not let you off their clutches/power trip.
I have never heard of anyone credible successfully getting their baptism deemed invalid.
Sorry that you are in such a situation.
Zing