OK, so to write up the rules for a “Foolproof” skill check that P-47Thunderbolt proposed, which can apply generically for any check not to just this house-rule:
Foolproof : when the Ability/Proficiency pool for a group Stealth check is expanded to 6 dice , it becomes a Foolproof stealth check that always succeeds (and need not be rolled), unless the GM decides to spend a dark side destiny point to force the roll with 6 dice. The spent destiny point does not change the dice pool, but allows the roll to be made, which the GM may decide to do if they deem there is still a decent chance of a failure or at least a Despair.
Yes, mechanically this is the exact opposite of Impossible checks.