I'm having the same issues as you described in the original post, with the same board driver (5.6.1) and arduino_builder (1.8.5).
What exactly do you mean when you said you're using embedXcode now? I installed it, made a new project, and tried to build it (following https://www.youtube.com/watch?v=b4lnz6ixvgU ), but the build failed at line 26, and opens up a browser window listing the boards and IDEs I dont have installed. Would I need to install everything else as well in order to get it to work, or am I missing something else?
The buildtime error says "MSP-EXP432P401R board is unk