I think, just being the first employee and the "rock" for other engineers (even though most have quit) leaves me feeling like I shouldn't just leave everyone high and dry.
But do you really want to be co-owner if the founders are constantly arguing?
You are your own person. If it's time to leave, it's time to leave. I value loyalty, and if you first want to sit down with the founders and tell them to start doing their job, then do that. But if their mismanagement is destroying the company, there's little you can do to stop it. Unless you take over. Are there any outside investors? What leverage do they have?
I can't see any way this company is going to succeed. I wouldn't even think about staying without engineering a change in management, which it sounds like would require ejecting the worst of the 2 founders, assuming the other isn't also inherently a non-stay the course micromanager. And "that trick never works".
Note also their response to the this catastrophic, frequently company ending debacle; if they don't take it absolutely seriously, and as their fault, leave, the company is just not going to make it. "You can't fix stupid" is I think the maxim there.