Now, if you haven't yet, now is the time for you to go see the situation in person.  

We would want to use it for personal use probably one per year. 

Do you sell / buy real estate in Mexico personally? 

This is so I know if I should keep the person I currently have.

I personally would not invest in detroit as historically its not a appreciating market and for cash flow and I invest primarily for appreciation - I go off the location location location mantra. 

One-person rental units are in the highest demand, and co-living directly appeals to this market.

Personally, I wouldn't offer SubTo or a Wrap, For reasons I've written about fairly extensively. https://www.biggerpockets.com/forums/70/topics/1227199-the-m...However, I would sell on "Lease Option" and that may work for you.

Tell me - someone selling their home to your son, Subject-To - is this a person who has made good life choices? 

I was thinking if I can talk to my neighbor about sharing the cost (I, the landlord, personally provide labor and my neighbor provide material (e.g. fence pickets)).

A contract is only as good as the person enforcing it.