If you are on the mid-level with only one key, you are in trouble. There is a way back down but time will be against you.
On reflection, it is feasible to build such a route as an Adept... it's just not worth it. Aside from anyhting else, better to get the save after you've wrapped the lower parts up.
As for losing to Pax with an Adept- it's never happened to me.
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Yes, there are a lot of threats the Marauder gets a break with using a gun. As I mentioned, he is actually generally safer. But it is some of the IMPORTANT bits he has a slightly harder time with, the bits you HAVE to do, whilst the other classes can plot a path around the stuff that would cause them trouble. This is always the fate of all-rounders.
I learned the game with a Marauder. Based on his work, I knew exactly where to go with an Adept as soon as I saw his ability, as I went to the bits that had bugged me as a Marauder. Then you just walk through nearly everything as a Brute.