In NYC the demand is lower in the winter but the supply is also lower so apartments rent fairly quickly still (typically within a few weeks).

That's the key component that is usually missing when I hear other entrepreneurs/investors say they don't like QBO.Something else to keep in mind is your entity structure and how your entities file tax returns.

Adding rooms does create the potential for more conflict but carefully thought-out floorplans, designate wet and dry storage spaces, and provide supplies.

In case helpful the below covers more about how DSCR loans are structured. 

Would you have any recommendations on how to structure this private money loan that makes sense for the lender?  

Keep it simple versus some of the esoteric structures being pushed.  

In my professional opinion- supply currently exceeds demand throughout most of the city.

Are you going to supply the down payment and how much equity are you willing to give them?

Agree with @Chris Seveney- people LOVE making their ownership structures so complicated when you can mitigate risk in so many simpler ways. 

We would handle the guest communication, coordinating cleaning, minor repairs, stocking supplies, calendar management, etc.