The rehab has been paused so I can’t even rent it 

So even if you had a buy out agreement negotiated up front and execute that buy out agreement, the bank would still require him to be guarantor unless something drastic changed on your side financially.The best scenario here would be to purchase and rehab the property using a hard money lender or bridge lender.  

This can give you flexibility for purchasing or even renovating the next property.

I would say that even more of our owners don't see the interior of the property for years after owning it. 

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.

Many European cities have existed far-far longer than the USA has even existed.

Most lenders will let you use up to 70% or sometimes even less.

The two value propositions are not even close.

Platforms like Zillow, Realtor.com, or even tools provided by BiggerPockets can help.

I think your questions are a bit to general and you are going to get even more general responses.