Not experienced in this by any means at all, but I still am interested in what are the legal, social, and professional responsibilities here. Legally, you are

Not experienced in this by any means at all, but I still am interested in what are the legal, social, and professional responsibilities here. Legally, you are entitled even though the market is no longer the same as which you were brought to help in. Professionally you have put in lots of un-tallied TLC. Socially, there seems to be no effort for an amicable resolution. On the business side it doesn’t make much sense for you to continue with the company if this is not your area of expertise. So you should taper off the position of founder and become a silent investor. Do not budge on percentage. It is your right. If the other person does not accept this, then the option is to dissolve the company. Keep all assets as is. And license to new entity for royalty or one time debt. If you want to continue with the company, negotiate a position that is optimized for what you can do. And remain shareholder. And board member. Either way, assess the true value of the company in terms of current potential revenue, future growth, and future risks. Use that as as premise for negotiation. And set aside a BATNA. A best alternative to negotiated agreement. Don’t focus on the torch and burn scenario even if the other person insists is a possible outcome. (All this comes from someone who doesn’t know a single thing about this other than how businesses merge, split, and dissolve.)