As far as the undated 20p goes, I believe the RM issued a letter to banks and post offices for them to return any bags/rolls of un issued undated coins, as suggested it would be impossible to remove those that were already out in the big wide world. That said the RM are actively removing Bronze 1p & 2p coins for recycling, so they must have a way of "filtering" the coins in circulation.
The "Underwater" Aquatic Olympic 50p were only issued on cards, the dies were recut before coins destined for circulation were struck. There was a rumour at the time that all of the "underwater" 50p coins were issued through the same London dealer, but I have not seen this confirmed anywhere.
Ian..