I'm wondering how much the appraiser might subtract from the ARV if I don't include Central A/C.

The combination of the tax return filed income and 75% of the gross lease will be added together and the lender will take that "number," to subtract it from your monthly PITIA (prin/int/tax/ins/assessments) payment.

If he's got 15K equity in real estate, the lender will subtract out the closing costs of, say $5000 at he'll get $10,000 at closing.

I use my own, I use real numbers from our STR's, add your PITI, subtract from the annual gross and divide by your downpayment + rough closing costs (didn't include anything for repairs or upgrades, assume its 100% turnkey). 

Subtracting from $16,800, and dividing that number by $92,000 I was looking to CoC return 13.7%.

It's backward because the appraiser said he'll determine the current value by subtracting the work that needs to be done.

So you'll have to add up the amount currently owed on those and subtract that from 75-80% of their total appraised value to determine how much cash you will receive. 

From there you subtract the depreciation amount to get your taxable income (likely negative), and that will flow through to your personal tax return.

So in your example, 70% would be 210k and then you'll need to subtract the rehab costs.

Subtract your fee and that's your offer.