Disallowing Mission Restart

Is there any way I could avoid letting the player push the "Restart Mission" or "Lower Difficulty" buttons in the Menu? If they do, my triggers will basically destroy all their progress in my campaign...

You can prevent that by using dialoge boxes using a "fake" defeat. Just get rid of the ability to restart of lower difficulty with your new "defeat box".

But the player will still be able to open the menu during gameplay... you know, the menu you get if you hit F10. I thought of the defeat thing, actually... if you do a "Victory for player" action with no next level, will it send the player back to the campaign screen?

Ive tested that... the answer is yes, it will send you back.