I think people misunderstand couple of things. "You can end mobilization" means that you go to domestic advisor and set it back to normalicy (after at least one peace is declarated). However, it sets back to normlity automaticly if you declare peace with everyone.
My guess, that if you mobilize in peace time and declare war you can set normalicy back (it will not happend automaticly). However, it needs testing (can not test it myself right now -- no save file of game around Nationalism and later).