Hello Guillaume,
Great question and I actually like your rule...but not for tournament poker. I have been lobbying for a rule that would prevent any player (in for all bets) from mucking his hand. This is the only way to guarantee that the pot goes to the rightful winner.
What makes your situation interesting is the fact that a player, that folded earlier, was able to retrieve the discarded hand before the dealer mucked it! Current rules allow players to fold at the showdown (as long as no player is all-in

), so the dealer should have instantly mucked the hand which would have prevented the messy situation that occurred.
Even though I like your house rules for the situation you explained, as long as the discarded hand did not "touch" the muck, and the cards were clearly identifiable, the best hand should get the pot. That's how I see it.