In my screwing around trying to get my save functional, I ended up uninstalling Diplomacy and Warband both, then after reinstalling the mod I launched the game and started up a new save without transferring anything from the native file to the Diplomacy file. All heraldic armor and shields were completely fine - so it must have something to do with either the factions.txt, scenes.txt, item_kinds1.txt or troops.txt that gets overwritten to make my save loadable. I'm unsure what to do next.