I'm an investor, real esate agent, and house flipper, I've never found it so difficult to price a home in its current condition or a projected ARV. 

So you have to hope that you can do that fast with minimal downtime.Without knowing the insurance, taxes, condition of the house and it’s mechanicals it’s tough to say how great of a deal this is.

On a positive note, those fees are only a few thousand dollars or less so even if they are forcing a buyer to pay this fee, it should not be a deal breaker depending on the duplex condition and price. 

Very tough to find with current market conditions in nearly any location right now. 

So, the calculator doesn't help determine condition of property.  

If we're kicking them out, we explain we're doing it per the conditions of the loan we got and wish we didn't have to...

To determine if the expense should be classified as a capital improvement or a deductible repair, the context of the expense is very important.An example is, if an item of expenditure is part of a general plan of modernization, renovation or rehabilitation to equipment or other business property, it typically must be capitalized even though on its own it would be currently deductible.According to IRS Code, expenses that you must capitalize are those that:Materially increase valueSubstantially prolong useful life (including replacement of deteriorating assets)Adapt the property to a new or different useHowever, you are allowed to deduct expenses and fees for routine maintenance and repairs that help keep the property in efficient operating condition.

You haven't described the location or the condition of the home at any point during those last 10 years, so I'm left to speculate, but it is very possible that the sale for 69k could have been a very motivated seller.

Then out of nowhere TWO MONTHS LATER they show up on Jan 2nd from their lawyer stating that "my client is ready to close by Jan 5th for a credit of $20,000 and my client believes the contract is still open and valid and my client has reinspected the property and is satisfied with the condition." 

Generally comps are not exact as this is single family homes so condition or layout or something is different and AC is just one of those differences.