I investigated, tested humidity and moisture and found no problem.

Dallas is an exceptional market but it's going to be very hard to penetrate the fix/flip market with really no experience or boots on the ground.

High humidity, high moisture, and termites.

-Make sure it’s zoned for two-family dwellings if it’s intended to be a legal duplex-Min ceiling height is 7ft from finished floor-Obstructions(air ducts, piping, etc..) can be no lower than 6’4” -Any wood framing that touches masonry(brick, concrete, floors and walls) must be treated lumber or divided by a moisture barrier-Every room must have an egress window if it is considered living space (bathrooms, closets, mechanical rooms are not living spaces)Great idea and best of luck to you!

This includes semi-annual HVAC cleanings, re-sealing roof penetration, changing air filters monthly, etc. 

Also what they are doing from inside the attic is checking to see if there is moisture or signs of water damage.

It did its job decades ago but so did flip phones.Architectural shingles look a million times better.Here the cost to source 3 tab is more than architectual.Nails should be long enough to penetrate the sheathing (unless in exposed finished areas)You have to account for sheathing 5/8"-3/4"Roofing 1/4" per layer minimumPlus any underlayment (ice and water + starter shingle)I would most likely use 1-3/4" nail for 2nd layer architectural.Most roofs old or new are vented somehow.

Concrete floors are permeable unless the pour was done since 1978 and the contractor used 6mil visquene as a moisture barrier over the gravel. 

And then you’re likely also getting heat penetrating into the attic.

I guess it depends what kind of flue pipes they are, if they are of the triple-walled variety then they can actually come into direct contact with combustibles so that might mean it's okay, but then again if the flue pipes were installed correctly there should be a fire stop/fire blocking at each floor/ceiling, which would be tricky to access in order to penetrate and if you you were to penetrate you would need to seal around with fireproof caulk to avoid violating fire code and I'm not sure how you'd do that inside the chimney.If there's any other option such as running it through the wall, on the wall, or even stubbing through to the outside wall, running up along the outside with EMT then stubbing back in and avoiding the chimney altogether, I'd do one of those options instead.