I have a few comments. First I have a number of USAT diesels that I run in consists.
I have an ABBBA PA set.
If you start pulling long trains or have grades approaching 2%, I'm pretty darn sure the decoder would not handle 2 locos, so it would make sense to measure your current draw on DC ON YOUR LAYOUT and grades and your trains, before trying to share one decoder among 2 of these locos.
USAT uses overly complex wiring to lights, because they use different voltages, and most are incandescent. That said changing out the PA to LEDs is a BIG task, and getting things like number boards to look better than a bunch of squirrels with flashlights is tough with LEDs in this case.
I would actually work on getting dropping resistors to power the existing lights, but you need to measure voltages on the various lights BEFORE you rip stuff out.
The smoke units came in 2 versions, one a small boiler type that run on about 5 volts, and the other powered heater and fan, and they have their own regulator board.
My best advice is remove the Airwire, but leave the factory boards. Depending on your battery (I'm guessing you are going battery), using the existing lights and the REGULATORS in the factory boards would be easier.
Go to my site for more details on the PA
Greg