Ok, I had to change my original answer as it really made me have to wake up and think:
When you counter a spell the following happens:
"When a spell is countered it is discarded without effect, and you lose the mana spent and any action used to cast the spell (unless the spell or ability which counters says otherwise)."
Since the countering doesn't destroy the spell, on destruction effects can't happen. Now, I'm not aware of any way to counter the force sword cantrip, but if there is something I am missing then it would be gone forever.
As for Decoy:
1) You do not get mana if countered, same as above. The spell is discarded, not destroyed.
2) You would get 2 mana if Seeking Dispell is used, as it specifically destroys the card. Decoy does not say it must be reveled to return the mana, simply destroyed.
That's my final answer Regis... let's see if I'm wrong.
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