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

Provided I can establish a solid investment plan tailored to their financial needs/comforts such as debt ratios and income, they would be more than happy to offer me some financial control.Additionally, If I wanted to, for the sake of building my own personal wealth, I admittedly have the luxury of taking a loan from my parents.

To wait until, the actions of others, made things look "cheap". 

I personally use 40% but I'm also in a higher rent area.

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. 

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

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

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

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.

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