You're approach is very sensible, I will utilize it.

You could even utilize a hard money loan instead and have your friend cover the down payment and interest payments, while paying him interest over the term of his loan or at the end of the project.

COC returns for both are double digits as well from my experience as well.

Commercial loans do not report to credit bureaus so it wouldn't affect your credit utilization nearly as much as buying the property with a straight HELOC. 

I just wanted to add also that many syndications purposly utilize cost segregation to generate losses- but lot's of people can not deduct those losses in year incurred due to passive loss limits. 

This might also make it easier to justify separate meters or “utility chargebacks” if the rents get too out of wack. 

If only half the rent is paid, they are still 100% responsible for being late and subject to fees or eviction.Personally, I wouldn't want a married couple renting a room because that will result in an increase of utility use, wear-and-tear, etc.

The only other change will be repairs/utilities and they will be affected by inflation.

Side Question:I don't want to complicate my primary question with any more variables, but as a secondary, ceteris paribus question.... if the owner lives part time in the STR how does one write off expenses not directly related to STR operation, such as insurance and utilities? 

They have better collateral and you are able to utilize your funds for renovations instead of the down payment.