So, it turns out that the error is stemming from KK's NewInstances directory in some way; i.e., whenever I have saved new statics/mapdecals there, the game fails to load presumably because of the aforementioned "Triggers on concave MeshColliders are not supported" error. The reason why my clean reinstall worked is because I deleted all of the stuff I had worked on previously. This means that the mod(s) work just fine as long as I never quit the game, but I have to start from the beginning if I ever reset... not ideal.
I know I should probably post this in the KK thread instead, but I thought it might be of interest. I'm doing some trial and error now to figure out if I can isolate the exact issue that's messing everything up; i.e., looking through the KS-R ExampleBases statics/mapdecals I know load without incident and trying to figure out what's different about the ones I'm making. So far, all I got is that mine are on Laythe rather than Kerbin. That can't really be the problem, right....?
EDIT: I've since isolated the issues to KS-R statics only -- existing MapDecals and GroupCenter cfgs seem to load just fine. Now, for the long process of trying to isolate which static(s) is/are causing the problem...
EDIT 2: I've identified the problem static as the 1700m runway from KS-R. I have no idea why, but the game loads fine once I remove the one I previously placed from the NewInstances directory. My only theory is that I told other statics to snap to it at one point and that messed everything up. I'll try placing another iteration of that object without that option checked and see what happens.