They probably didn’t test the power-up sequence at -60°C and in a near vacuum atmosphere.......................
I think they did, in a big vacuum chamber. But they couldn't simulate the exact solar conditions they'd have on the Martian surface, and the temperature soak conditions, as you alluded.
They probably left it with some items "to be determined," knowing they could reprogram it in situ. I'm sure they had a list of parameters that they could settle before launch, and another list of parameters that might have to wait till they could do site testing.
A real nail-biter, I'm sure. If they can fly and recover it without tipping it over, it will be very impressive.
I think the Gil Scot Heron song Whitey On The Moon pretty much guaranteed that U.S. Government-sponsored planetary exploration will be done by robots.