Recently, I had to pay $60 for an HVAC tech to simply tell me that the air filter needed to be changed on a 4-month old HVAC system in one of my rentals.

I like my contracts air, water, and BS tight!

If you make a tight cut in a door way and then the floor expands it will apply unwanted pressure and you could get air pockets under the floor also.

BTW whats a Air bnb unit?

In a space like real estate investing I think there's probably too much over-polished sales pitchy stuff out there, so I'd tend to air on the side of staying a little less polished, a little more authentic. 

My problem, the other tenant is an attorney and threw out the following:As pursuant to the agreement on page 1 of 6, # 17, Maintenance:“Tenant shall professionally maintain the Premises including heat, air conditioning, electrical, plumbing, and water systems, if any, and keep glass, windows, and doors in operable and safe condition….”

Heat is forced air.  

Be sure to get mold testing done after remediation and removal of drywall areas so you have a record of clean air following the remediation if anyone were to ask, but you shouldn't need to provide this information unless requested.

Did you show the seller in writing from a GC that it would take $90K in repairs or did you just pull that number out of the air to get a better deal?

If it's as bad as you say, you may have to replace the a/c unit and possibly the air ducts, just something to think about.