If the partner wants to get a bigger cut on future deals, they can save their money and buy into a larger percentage on the next deal.

Not possible, 85% max LTV on a purchase or straight refi, 75% max LTV on a cash out refi.

As others say, make it crystal clear in your listing that the "shells may cut a dog's paws, and we don't allow pets." 

What would a fair cut of the profits be?

They also know when to cut bait.  

A.I. says I’m cutting it close.

I'm not sophisticated as Dan, but if you simply do the straight math, it may be close to 10 years before you break even on $190k. 

This friend and I have done 2–3 deals together, and there’s never been a payment dispute or even a hint of cutting him out of something.

I would look into every and all claims about someone as well as reach out to his former colleagues (who are here on BP) - as part of my point was sometimes its easier to go straight to the horses mouth and see how they respond.  

There are regulationsI could go on, but if you learn this much, and apply it appropriately, you will cut down your sorrows and risk, considerably.And yes, there is much more.