I've determined that due to my somewhat unique funding that a DSCR loan may be my best bet.

I would bet there are several around where you are.

I wish them the best of luck, but I'll bet the execution of that business plans goes sideways and in then end the proforma gets trashed. 

I could be wrong, but the only way to make lots of money is to make educated bets and execute them.

I can’t lie, I felt a little jittery stepping in, kind of like I was about to bet on a horse race without knowing which jockey had the best track record.

I bet they would jump on it to save the money- or simply help them with social media marketing etc.

Based on the numbers, I am willing to bet this is located in 38126, 38106 or 38112.

Philly is your best bet. 

While BP has a rent estimator as does rentometer.com (both allow a few free uses, I believe), I agree with @Zachary Deal that your best bet is to seek out guidance from either the listing agent or a property manager well-versed with the area. 

If you're looking for appraisal value an ADU may not be the best bet, but if you intend to sell, my anecdotal evidence suggests in dense highly desirable neighborhoods, ADU square footage will gross the same as the main house.