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

If you are still not able to make it, you might have to go back and go through all your returns and property documents to figure out the cost. 

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

If your intention is to use subject to, to enter into a business with another individual, the best advice is to have an attorney draw up both documents.

With completed & documented rehab, no seasoning is available.

I'm going to have an attorney review my lease agreement, but all the documentation that I have came with my BiggerPockets MANAGING RENTAL PROPERTIES book a few years ago, pre-pandemic...and it's a lot.For those using KeyCheck for their MTR listings, did you also find that additional documentation was needed, besides the 3 key things that KC provides? 

While it’s frustrating that the tenant is still there and not paying, turning off utilities could complicate the situation and potentially lead to legal consequences for your "friend".In the meantime, document everything (the tenant’s non-payment, the lease terms regarding utilities, and their attempts to resolve the issue) to present in court, which may help recover the outstanding balance later.

Absolutely, the good news is that we didn’t buy it for the purpose of income.

The only mention of mobile homes I found is on page 13 of this document classifying them as “residential rental property.” https://www.irs.gov/pub/irs-pdf/p527.pdf 

After that, run the background and credit check, verify their income by reviewing the financial documents they provide, and contact their employer to confirm their employment.