G'day wobble,
From what I recall, declaring a baptism invalid, whilst not common, has been allowed in some cases - but I would not think it's a good move for anyone but the young! Leading life over many decades and then claiming an invalid baptism is , well, going to draw more attention to yourself than simply drifting off into the shadows, as it were.
BTW got your message - I'll email you later.
Cheers from downunderneath youse lot,
Ozzie