It sounds like the loan you're referencing for commercial properties (30-year amortization) relies on agency debt, where the loan balance would likely need to be over $1M.   

I have relied on craigslist and zillow in Clarence and usually fill a vacancy within a month, and Clarence is not very populated.

I relied on the reputation of the company and checked their figures and the neighborhood remotely.

I’m solely relying on Zillow, Trulia and Redfin. 

I really had to make a couple trips to get a feel and now I still really rely on my PM for neighborhood recomendations.  

So was Zillow actually relying on the active investors it had qualified to figure out the real world rehab numbers & ARV and thus come up with the "instant offer" more than their algorithm?

I put my career ahead of real estate investing because I rely on it to support my family.

You can not rely on your lease languageas it may be in violation of state regulations.

As for credit score,  I don't rely on set number but more importantly patterns that show up on the credit report. 

Your obligation is to your sons and yourself and this tenant’s behavior will cost your sons money; they are relying on you “feeling bad” to prevent you from forcing them to do what they should.