That means you have not installed Halo:CE from in the game, steam does not install the game just the dashboard update for MCC
alt-f4 and re-launch game. I have yet to replicate this myself so unsure at the moment.
I managed to replicate it by saving and quitting to the menu and launching back into the mission
I can confirm this, as well.
you can use both. you just have to rename them in your trainer folder, so you can use both. just add Reach to Reach trainer and vice-versa