We are their top choice when it comes to sealing a deal.

SAME as ^...Also, I have a buddy I just helped house hack a duplex in Raytown...he was going to try 'skim coating' over laminate with concrete, and then sealing it.

Common culprits include poorly sealed wax rings, dry traps from leaks or non use, and localized clogs that vent sewer back into the room. 

You have to remove the effected material or seal it with Killz and other products....regular paint wont do it.

They were willing to front the cash quickly, which was crucial for sealing the deal.

Are they spray foamed and caulked to seal leaks around them?

They were willing to front the cash quickly, which was crucial for sealing the deal.

There are other forms of winterizing like insulating plumbing, sealing door/windows with rubber gaskets, installing storm windows/doors, setting up heat lamps or other supplemental forms of heat in basements, etc.

The tech tried replacing the door seal, and it still had a very slight leak, so he recommended a new one to the company.

The idea is that if a conservative bank will grant a mortgage, it's a seal of approval for the project.