Another thought - using the "initials on the back" method, you wouldn't need a duplicate, making this trick all but impromptu. You could also eliminate the force and control sequence.
Then all you need is a more effective "vanish". One idea (seeing as you have to work one handed) is: Card is placed face down onto face down half deck - the secretly initialled card is prepped under the table. If you had an X card palmed, you could add the card to the deck as you slam it down to the table...when you lift your hand the initials will be gone - and the other initialled card will be showing under the table. Bring it up and either triple lift, or execute the underground change, to reveal the selection. Palm off the dodgy card.