Interesting, thanks for this. I didn't explain myself properly earlier, but that icon right next to the platypus one (on right) is the second icon I was asking about (with the text exec on the icon), and those processes "RSConvert_GUI" are both related to that (that's the actual program running two processes). 99.9% seems a bit excessive though, yes I think I need to compile a debug version to get some more specific data on what's going on. It clearly goes into some sort of infinite loop hogging all the cpu, and if it doesn't even show the GUI it's probably related to the GUI framework. What osx version are you running by the way?
One thing you could try to do is to run the RSConvert_GUI from terminal. In order to do this, you should open the terminal to the folder where the app is located and then:
cd "Rocksmith 2014 CDLC convert pc mac.app/Contents/Resources"
./RSConvert_GUI
That should keep the terminal window open and if we are lucky, it might print out some additional information about what's going on. Anyways, even without that you have already provided me some very useful information. Thanks again! I hope I can make this work, as I personally really like this new version much better than the earlier one.