On one of them I verified it was going into the storm sewer by putting a hose into it and then Listening at the sewer grate on the edge of the curb.

So my advice would be to verify the rental market for homes in the same price point as yours.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

Lenders will want to verify your income, debt, and credit whereas on a Hard Money loan, lenders will qualify you based on the deal itself and your experience.  

I found out when I decided to email her a question at work as a way to verify her identity.  

Within 24 hrs of the negative review, Relay reached out to me and requested recent bank statements from my source account to verify.

What documents do they require, what credit scores do they allow, how do they verify previous rental history, etc.?

I would connect with mortgage servicer to verify.

I would verify this, but this was the rule in 2020.- To add on to what @Sarita Scherpereel said you are still taking a risk with the city on the nonconforming unit even if you get the loan

Verifying last 2 years of rental history very important!