@Ross YeagerSome quick thoughts: Technically you might be violating the Pennsylvania law that states something along the lines of only salespersons and brokers can manage properties for others.

After that, they technically owe capital gain tax, however being an elderly couple, they may not have enough income to actually have this tax!

Reason being is that youre technically still in escrow, and you cant really be in escrow with multiple buyers for the same house..  

(I know this process can take 30-45 days but I may be able to get the time with the investor)I can offer a 10% return on their investment with a 5-7 year hold period (after annual custodian fee – would technically be 9% on 30k investment), which would allow me to build my equity to 40% and cash them out with their 20% initial investment.

No legality issues as far as I know and when you disclose this when you sell you can say HardiePlank as it is what is technically on the exterior of the property. 

So long as you stay away from consumer loans, Texas is pretty friendly when it comes to letting businesses and sophisticated parties do what they want. 

I’m specifically inquiring about how to explain to them the transition from the current rent collection strategy used by the PM company to the rent collection tool offered by Cozy (which is not technically a legal lease agreement).

Account Closed Technically yes but it would be up to the bank/lender to accept. 

Most sellers are not running a sophisticated operation with duplexes where they have professional P&L's & things of that nature.The reality is most of these assets are owned by mom & pop landlords & they are going to vary based on who is operating them.

Also can I just use a standard mortgage calculator for the loan terms being that I am not technically refinancing a previous loan?