WOW ... This situation (together with the other one : the last player at showdown with cards obliged - or not - to show is hand) shows that at SHOWDOWN: we must all help the TDA to create clear rules!
So what about this first conclusion (following - or not - some of your advises) :
At showdown a player (at is turn) says "I fold" and throws his WINNING cards in the middle faces down (but not in the muck so clearly identified):
A) If he finally realises that his hand wins the pot and turn it faces up: HAND ALIVE WINS!
B) Some involunteer element (air, unvolunteer movement of anybody) turns the cards faces up: HAND ALIVE WINS!
C) The dealer (buy error) ask the player to show the hand or directly turns it faces up himself: HAND DEAD! (RDW reference)
D) Any player (but not the actual winner) ask the player to show the hand (this can be exceptionaly accepted) or directly turns it faces up himself (a crime): HAND DEAD!
E) The actual winner ask the player to show the hand (personnaly I allways refuse it: this is only curiosity) or directly turns it faces up himself (a crime): HAND ALIVE WINS!
... So my (humble) actual responses to thoses two showdown problems are:
1) Must the last possesor of a hand at showdown show it to win the pot? ... NO! (in general)
2) Does a folded winning hand et showdown wins the pot if returned by someone else? ... NO! (in general)
My general spirit (when facing a "no-rule" situation) is to RESPONSABILIZE the players. This rude and beautifull game had been invented buy rude guys from Texas ... I try to respect their memory and (only in "no-rule" situation), I prefer to favor the player FIRMLY HOLDING HIS CARDS IN HIS STRONG COWBOY HAND (lol) to wait for my decision than the FOOL OR ANGLING PLAYER (lol again) who can't play poker ... or tried a tricky move!
Come on: the name of this game isn't it "HOLD'EM"? HOLD YOUR CARDS MAN!

But once again the day the TDA choose a CLEAR RULE: I will follow it IN THE BEST INTEREST OF OUR JOB!