You're  also multiplying the amount of equity build up from the principle paydown of the rentors. 

Discussed with her that this may or may not hold up as an objection and asked what we could agree on.

There are other reasons for not trusting your gut besides it being wrong most of the time.You MUST have objective standards for who you rent to with NO exceptions.

But you already own a property and maybe you can maintain objectivity way better than me. 

Got a new one in Sept and almost half my payment is going towards principle already.So this isn't a which is better comparison, it's a how do I get a 30yr loan with a partner question. 

Other instances they stopped paying and the fees and interest collected goes up quickly including legal etc.I hold the note on a property that the unpaid principle balance is $65k but it has $85k in late payments, fees etc.

In the instance of a BRRRR investment, and the owner is able to cashout his principle up-front, would it seem fair that (assuming a 15 year note), I would get the same proportional share on the property once it's free & clear in 15 years or if the house sells?

Think a list of qualifications that applies a weight to each trait:Credit Score - XPets - XIncome - XPrevious recommendations - XLength of proposed lease - XDeposits - XTransunion gives you a go- no go decision but the above may help you grade the quality of each potential tenant while being objective.   

So I have stuck to that principle and it has worked for me even in my next home (personal residence in CA).The obvious choice (Call it CHOICE #1) would be to look in the same Chandler area and see if there are any deals there.

You are correct, my objective is to buy and hold.