Total posts : 45366
The TH transmitter and ATU were certified to operate together and presumably it would be legal to elevate the ATU using the transmission line but it gets a bit fuzzy since certification allows the manufacturer to sell the equipment but it doesn’t grant the user any special exemptions for use. If it is legal with the ATU on the ground using the transmission line then elevating the ATU should be essentially the same thing.
Most likely driving the ATU with another transmitter and a transmission line may not be so under 15.219.
Any antenna, ground, and transmission line configuration is legal under 15.209 which has field strength limits. It is generally accepted that an optimized system under 15.219 will provide greater range than one under 15.209 but, as you reported, decent range can be had under 15.209.
From my experience trees and bushes near a ground mounted vertical will affect (reduce) the radiated signal as observed by a monitoring antenna nearby however the effect is small and no reduction in usable range resulted with my station.
Buildings and obstacles which are small with respect to wavelength do not block the signal so the typical residence or small business buildings are not a significant factor. The signals may be attenuated inside the structures but externally they diffract around the structures.