You have to think of this as a marriage of sorts. You're going to be spending loads of time working together under often very stressful conditions (which will only exacerbate whatever red flags are currently there). I don't see a successful outcome of this for you regardless of whatever discussions or agreements you put in place.
If you like this space, why not form your own team with like-minded folks and build your own venture? This is so early on that your starting at roughly the same place. As a developer you can probably throw together a quick demo of something to demonstrate an idea and be further ahead than where this other company is now.