I confirm that andy, they do die in war and serve in reg army.
Andy we kinda can stop them from being used in reg units.
@burns, transfer what you don't need to account or other slave/main that is not going to go to war. this way, you only have to add what is lost during war from the mobile units, use the screen andy said:
he exact numbers are available under the Army menu option --> general data.
Use data from there to give you the number to move them and remember to check after war for a couple Gmos to make sure you don't lose your mobile units, a pain to do since so many steps, but it does what you asking for. Minimize the loss of Professionals, when out of war you can always move them right back or move mobile units back to country.
@ANDY: it would be nice option to allow them to be used as workers as what they would be if not in the army as a choice and listed as such in the workforce also, kinda like what the army does with them... but still be able to use them as professional soldiers when we want to. Or have a reserve limit to set on what to keep as reserve out of units and be nice to have them still being used in workforce.