Unselie, you say its undoable because of pieces in the Story being moved.
The two that come to mind are the Magellan Jumpgate and the Leeds Jumphole. Not only that but the changes to Leeds itself come in to play at some point im sure.
If they play The SP Campaign off a completely clean installation, no mods at all. They shouldnt have a single problem with the campaign.
As for changing the difficulty of the actual campaign even with an unmodded installation.
Go to Documents, My Games, Freelancer, Open PerfOptions
Edit the line that says:
DIFFICULTY_SCALE = 1.00
2.00 being twice as hard
1.00 being normal
0.5 being twice as easy
and every other number respectfully.
I await the day someone posts a complete video where they completed the singleplayer campaign with a difficulty of 10.00 (10x as hard), from start to finish without cheating.