Underwriting a tenant-buyer: current payment record or a lender's written path to qualifying?

I'm building the screening file I'd use if I ever offered a rent-to-own on the house I'm about to close on, and the standard tenant screen doesn't answer the question I actually have. A regular tenant only has to pay rent. A tenant-buyer has to pay rent and become mortgageable inside the term, and those are two different underwrites sitting on top of each other.

One school says weight the payment record and nothing else. Twenty-four months of on-time rent, verified with the prior landlord rather than pulled off a form, plus a bank statement that shows income arriving in a pattern. The logic is that a person who pays reliably will keep paying, and credit scores follow behavior eventually. It also has the advantage that you can verify it yourself in a week.

The other school says get a mortgage professional to put a written path in writing before you sign anything. Score today, score needed, the specific items blocking approval, and a rough timeline. If nobody will write that down, the term you're about to agree to is a guess. The cost is that it slows the deal by two or three weeks and some good applicants won't sit still for it.

A third position I've heard is that the option fee size is the real screen, because someone who can produce five percent of the price has already demonstrated the discipline. I'm skeptical of that one, since gifted money looks identical to saved money on a bank statement.

What's actually predicted whether the tenant-buyer closed?