I know this is generic, but may help develop a mindset when negotiating.
I have always found it best that at the end of the day there is only one boss (typically no 50/50), and there is a clear governing document (operating agreement for some entities) that outlines each members roles and responsibilities. It would also outline how to end the partnership amicably, and for non performance.
If i understand your current situation correctly. I would push to have the new investor as simply that, an investor and not a member of the company. I would likely want the investor as silent as possible (maybe you don't). I would think a promissory note, investors agreement, or some other legal document could be structured to meet you needs.
With that said I have seen big investment groups create separate companies for project specific investment. They then sell shares to investors. Everyone shares in the profit or loss, the group I have in mind does a single exit at the end where everyone gets paid. They limit this to qualified investors only, not sure if that is an SEC requirement, or an internal requirement.
Talk with an attorney.
Hope this small brain dump makes at least a little sense.