Mega Gengar ex works well in a dark box style deck, especially paired with Mega Sharpedo ex who can be your big hitter while Mega Gengar ex reduces the amount of prizes your opponent will take, enabling a favorable prize trade
You won't have to pay the commander tax to activate that ability, and activating that ability won't increase the commander tax to pay later.
If i can add some info on some: The zukan cards are Bandai Carddass, zukan just means pokedex, its not the name of the company
Product names, allocation numbers, international versions, pull rates and final card lists can all influence availability and market behavior
As long as players are using opaque sleeves, such that no one can identify these cards out of a stack of normal cards, does it really matter