Withdrawing a paper after acceptance due to error
I would contact the programme chair of the conference and ask for advice (perhaps suggest the options you have already given).  It could be that by submitting the paper you are under an obligation to attend, in which case you should seek permission from the conference chair to withdraw the paper.  To protect your reputation, at the very least explicitly state in the draft that lemma is currently unproven.   We all make mistakes, without exception, so we should all be understanding when they happen to others.