@OldTrees Yes, 'shuffle a card into deck' is meant to indicate that a new card is added to the deck at a random location. This was intended, ad dragtom mentioned, to differentiate the mechanic from rewind which adds the creature to the top of the deck.
It could indeed allow deck to contain more than six copies of a card, although controlling that would be a bit difficult since the creature picked could be anything that the owner has enough quanta to cast.
@dragtom I originally wanted to do just thst, but i thought it might end up a bit op wuth mal cell as you mention... I tend to er on the up side though, so I'll see if I can reword it it to function the way you mention.