Yes, in DevMode you should use mas Files only, when they only contain Files, that don't exist as loose Files in the ModDev Folder, at the same Time. And just to be clear: the Track learning ini Files also work in the main Game, when you put them as loose Files into the ..\UserData...... Car Folder. Is that correct? That is a good Idea.
That is correct. NOTE: In ModDev, I wasn't able to use my vehicle set even when COMPLETEL:Y packaged into MAS files and it was suggested that this is a known issue and stick with ONLY loose files. I suspect mileage will vary depending on what you have for files.