I think your "training missions" would be stuff at the academy that you're supposed to RP. That's about as safe as you can get. If you want to do something so safe that it barely does anything, it's better off not even being an option. Why?
On one hand, if it's really safe, but gives a higher chance of skill gain than academy training, and maintains skill each time you use it, then it'll easily allow someone to go to 100% infiltration, no problem.
On the other hand, if there's no skill gain or maintenance, then you're just wasting time and effort doing something that would barely be noticeable. Better off investing the time and money in the academy.
However, what could work is a skill gain limit to certain "safe" actions. This means that once you reach, say, 40%, then doing something like "sneak into the baron's sister's bedchambers", would provide you with no chance of skill gain.
