Should nonrepeatable missions be reattemptable on fail or abort?

I want the rewards for success to be real rewards, not inevitable conclusions to tricky levels. Once I design a bit more of how my plug will work, I will describe a scenario and justify why it is better that the player actually have a shot at screwing up.

I think minor missions, even i n major strings, should be reattemptable. The delivery missions in the Fed storyline are a good example. Major missions should change the course of the storyline if failed. The Rebel storyline has something like this: if you fail to deliver Mu'Randa to Kel'ar'y you get pushed int a side string where you eventually get a Rebel Cloaking Device. Unfortunatly, it is virtually impossible to fail that mission unless you delibratly dump her out, something most players would not do.

Yeah, thats the problem right there. How do you balance it so its possible for everyone to do, but trivial for no one? And how do you hint that there is something they might be missing?

Im thinking that there will be a mode in my plug (possibly a single mission in the bbs only of the starting planet) that will set a bit that gives the player OOC hints as to what to do, maybe hidden inside thoughts the char has ("It crosses your mind that it might be benificial to you if you just executed the official yourself. Your men have already have everything they need to know, and to turn him over to his officials would earn you nothing but devaluing the information you already have. Your finger wavers over the 'eject cargo' button. After all, it wasnt an accident that his quarters are just a sectioned off corner of the cargo hold").

Or, you know. Something to that effect. Make it a little more clear to the player that the action most beneficial to them might not be just to follow orders.