Ask your lawyer if it's wise to keep the letter stored in a drawer until they want to go public. There is chance that they will flop before that, but also a chance that they will be more "friendly" to make the due diligence happy.
I'm more expecting them to try and get out on a private acquisition instead though. They run themselves as scammy con artists. They're really good at marketing and selling, not building or managing.
Just an email from your lawyer to the buyer telling about your case might stop the process and will make the founders do a lot of (unwanted) work.
If I wanted to be _really_ evil I would transfer my vesting rights to a third party that acts as a debt collector before the mail is sent removing my person from the process (If it is legal and feasible - remember: I am not a lawyer)