I will play with instapasses

Yes, you can always just use rampages to set OMF to 0. You can also use Cheat Engine to set the memory value, although I do not know the memory address of the on mission flag.

You can also use the mission loader dll file to play around with instapasses, but it is a different story.

But the OnMission Changer is by far the best option for you, so I recommend using that.