b) because I can
Thaaaaat's a negative. You can hide the mission as well as you'd like, but nothing's going to stop someone going into the .vp, copying it, and loading it up in the simulator. You can add checks to stop someone launching it from the simulator, and the player can remove them just as easily. You can get into an obfuscation arms-race if you want, but all you're doing is making your mission more complicated (and introducing the possibility of bugs while you're at it); if a player wants to cheat into the mission, they'll, because they've got all the data there already.
...hell, even if you did find some effectively
unbreakable system, nothing stops the player from ~k-ing their way through the entire campaign.
Now, I mean, there's nothing wrong with having an Easter egg mission or w/e, but going out of your way
to hide it is an exercise in futility. The checks you have in place seem fine.