The 'dirty nest' problem is one I've seen mentioned, they've modeled thruster firings impinging on soil - not entirely sure of the results.

I THINK ( and I'm just guessing here ) - while there's pressure in the system, you can't let the hydrazine freeze because it would rupture fuel lines and then possibly leak out onto the surface. There could be as much as 10 or 15kg of it left on landing.

If you depressurize the system as soon as possible, you don't need to worry about the freezing so you don't need to heat the lines etc - which would have frozen up really quickly in the shade, up at 68deg North. Vent it out, render the whole system benign and you don't have to think about it again. That's what I'd have thought anyway.

Doug