That's the other thing. What does Jack of all Trades mean to each player?
You're right. Once more we should define the worlds we're using if we want to understand each other:
I see JOT as the capability to improvise and use unorthodox means to attain the desired goal when the orthodox ones are not available or they fail (on a practical basis, not on a strategical one). McGiver would be (IMHO) a good example of high JOT skill.
I like that aspect, but I also allow it to be rolled to avoid an unskilled penalty - Formidable, Edu, JOT, instant, fateful, safe.
On success, one unskilled task may be tried as skilled with a level of 0.
I never liked the idea to use it as a substitute for skills you don't have, but as a coadjuvant to the one you have or to the unskilled use, as, IMHO, it does not represent knowledge (that would be covered by EDU stat), but the capability to improvise (see above).
In any case, let me share the thoughts your post took to my mind (little more than random thoughts):
- Being a formidable task (15+) its use in this way is very limited, as you need a +3 modifier to have the slightest chance of success (and I guess it's not elegible for cautious task).
- See that you could use determination instead of EDU, meaning both the influence of reasoning to understand what you have to do (INT), more tan having previous knowledge (EDU), that you don't have, as you don't have the skill, and the daring to try and self-confidence(so determination and not bare INT).
- To use it that way, I'd make it easier than formidable (probably difficult) but uncertain, making total failure represent that the caracter believes he will be able to do it (so commiting him to try), but using the untrained modifier.
- Can a task be at once fateful (a mishap is guaranteed if the task fails) and safe (mishap is never damaging, and on a fumbe mishap is superficial)? I see both labels as quite contradictory: on a fail there's a undamaging mishap

confused

, and even on a fumble damage is limited.