I listed the home on Zillow so they could do a background check, and the woman is the only one who has done it (guy says he will do his shortly).

Can I still run a background check on them at all? 

I know there's no cartel but the way it works is "technically" really like cartel.

I'm sure you are already beating yourself up about not doing the background work.

While I understand that I come from a fairly long and hardened institutional background, I am absolutely astonished by their apparent commercial success.  

Good stuff - you are pretty spot on - technically people max out under conventional loans at 10 - but oftentimes its with fewer properties as people run into hurdles before hitting 10, some of which you mention - wanting to diversify strategies, multifamilies, needing LLCs etc.I think you are on the right track for DSCR Loans - I always say that DSCR is really perfect for people in the 5-50 property range - typically conventional is the best fit with your first few, and then when ready to make the "jump" to scaling bigger and faster - DSCR is the best bet.

Technically, if you put 50-75% down almost anything would cash flow but then you would be cash in heavy.

So technically it will depend on the zoning if it is considered commercial.

That nice looking patio in the background of one of the photos ... can they sit out there?

I did do a background check, credit score, and talked to the previous landlord to ensure they are responsible.