but I have now talked with 2 firms, and both quoted me a flat rate for each of the 3 above categories I listed - and that was with just quick surface level knowledge of my situation.

Not just spray Lysol, but wiping down all surfaces. 

Does the speed at which the orbital moves degrade the purpose of this step (pitting and grooving the surface for the paint to adhere to?

And besides cleaning, you have to disinfect every surface. 

Depending on the type of tree most feeder roots are fairly close to the surface, and given that there's a house on one side and a driveway on the other I would guess most of this tree's root system was in the dirt you just covered with pavers. 

I bleached every surface twice , yellow stuff just oozzing down the walls , the white ceiling fan was dark brown .

On the surface, his real estate investing principals may seem idiotic...but I'm pretty sure he's far from that.

Covid19 has been seen to exist in surfaces even after 17 days.

I can schedule the checkout for a few days after they leave so that coronavirus on surfaces is no longer an issue.

(I say if needed because it isn't needed for the coronavirus specifically, as it only lives for approximately 24 hours on surfaces.)