What you got is the blister leather version of the jacket, which tends to be a stiffer lambs leather than the ones Rick Owens has used before. I know that the FW blisters were slightly thicker (because I have one) and not as slouchy as some of his previous ones. So they would run slightly smaller than your normal size. I haven't seen the Spring blisters yet, so I don't know if they would be a little thinner. Jackets tend to stretch out in those areas where the leather is strained so the area you're talking about should loosen slightly, although in my experience the armpit area won't stretch as much as the back for example. Even if you sized up and the jacket fits perfectly, I don't think it would really stretch (because it doesn't have to), so I wouldn't be concerned about a bigger size getting even bigger. Leather only adapts if it needs to.
Is there any way you can get the size 42 from NAP, compare it with the one you've got, then send back the one you don't want? It's just impossible for you to compare otherwise. I know the ladies here are helpful, but not every US size 2 is the same, i.e. girls have different shaped arms, bust, waist etc., but can still have the same general clothing size. And if you got both jackets in, you could combine them with different outfits to see what would work. Some girls only wear a t-shirt under their jackets, others prefer a long-sleeve or a thin jumper. Sizing is such an individual thing, you're best trying it out for yourself, especially if it's a jacket with a high price tag you'll be wearing the rest of your life.