Flashback anon here again, ran out of space. Well, Syaoran always could harness the cards powers, as he was one of the candidates to become their master, as for changing alliances well, it’s better than vanishing into thin air? The secrecy could be so Sakura won’t feel betrayed or forced to recreate an identical deck of cards. So far the new ones have been similar to the old ones or based on a person, but not all of them which hasn’t raised eyebrows yet.
Thank you very much for the concise list of clues!! I can see where this is coming from. I thought that by turning all the cards into Sakura cards they were completely under her control, but magic has proven itself time and time again in this series to be more fluid than rigid. If this is the case, I will accept it under one condition: there has to be a REALLY GOOD reason for him to keep this information from Sakura for so long!!!