Sub Zero temperature my 5D II mirror failed. It makes a good argument for mirrorless.
Batteries have a problem in cold, but I have no experience with the M5 battery.
The M5 has a problem eating batteries anyway. Would you be able to charge on the ship (I am assuming ship)?
If I were somewhere on vacation I couldn't charge for a week with my M5 I would take a half dozen batteries. Maybe more.
I haven't been but Antartica is on my list. There are many articles on taking the trip. A common theme is condensation and failure because of moving from hot to cold environments (ship to outside). Like you say the M5 is less robust, I would worry more about it making the trip than the 7D II. I have this part of my plan pegged for when I go, take every body I have for backup. This trip would give me a reason to buy in a 1Dx II for the cameras robust nature.
From the trips I have looked at $20 to $25k would get me there and back with the wife. It would be a once in a life time trip, buying a backup body and better lenses to take would be easy to justify.