Many lawyers will give a free consultation, have you considered consulting one to explore your options?
However, if you tell your former employer that you feel 11/12 of your cliff in equity is fair; otherwise, you'll pursue a law suit, my guess is they'll pony up without you having to take this any further.
IANAL and this isn't legal advice, but I want to comment on your wording. You should word the "threat" as saying that you may pursue a lawsuit, not you will.
The difference is really subtle to most people, but I once heard a story, on HN in fact IIRC, about someone who was sued because he said he would sue a company and then didn't follow through.
The company's claim was that they had been operating under the assumption they were about to be sued, and had incurred legal fees and lost productivity as a result. The company won.
Basically, word it as lawyer-friendly as possible. You want to let them know that you're seriously considering options that they would prefer you didn't, but you don't want to legally commit yourself to a particular action.
Again, IANAL, and this is based on my faulty recollection of a comment I remember seeing a while back.
Lawyer != lawsuit and it doesn't have to make things confrontational. It is just smart. If they are smart they talked to a lawyer about how to let you go.
I strongly recommend that you talk to someone like him. An experienced lawyer will be able to put this in perspective for you in short order: industry norms and expectations, legal options, negotiating tactics.
This could go all sorts of ways. On one memorable occasion, this lawyer wrote a short letter and got me $20k in wages owed. On others he's advised me to just walk away.
I'd also suggest you consider cash instead of equity, especially payment for expenses. You may prefer a longer runway (or a month recovering on a Mexican beach) instead of more lottery tickets.