It says it will help a leaders next roll. Is this only valid during the current turn or if you play this stratagem on a Leader only to change your mind will this Shadow Spy will be attached to that leader in future turns until the leader has made a roll. Which perhaps might be in the background if they are just going about their normal duties, rather than you activating another stratagem card with that leader. I suppose all leaders has rolls going on doing their jobs, so maybe then that Shadow Spy cannot be used by the player again in next turn if the player was hoping to play a diplomat stratagem card for example?
Shadow spy adds a +40 covert ops skill roll bonus, not a flat bonus to any skill roll.
I don't remember if the bonus persists until that leader actually rolls the skill or if it vanishes the turn after. Your fluff ideas are interesting and nice, but in the end probably irrelevant, mo harm intended.
If you want a diplomacy bonus, you wantShadow diplomat.