I do not think I would buy that car. Way too many questions. Have you verified they are giving you the correct vin? It really looks like that Denali does not have the ultimate package. You’re picking out the things you can see from the pictures, what about all the stuff you can’t see?
The auto start stop button is not really removable after the fact. If it was built without auto start stop, it would say so on the window sticker, just like all the other vehicles from this time.
Your gonna buy a used first year Yukon made during the covid/supply chain fiasco? Good luck. The trailer brake says dealer installed which is probably an aftermarket one located in a different location (maybe on right side of wheel?) or it could have been removed by previous owner who didn't tow. Same for running boards - this truck has 3-4 years of previous ownership...
The trailer brake controller does not say dealer installed, that is referring to the line under it for the illumination package. With ultimate package, this should have had the factory integrated controller and that one is definitely not removable.