1) How One Piece Card Games resource system works All players start with a deck of 10 DON
The deck could immediately win upon resolving Splinter Twin on its Exarch, so even though Splinter Twin did nothing until the turn it won the game and extra copies in hand would be dead, it was worth it
If you read between the lines, for 4 mana on the first cast, you get to choose from 16 cards in a full-sized Commander table that then can be cast to assemble your remaining combo or just find the combo pieces you need to kill them on the next turn with, of course, Demonic Consultation + Thassa's Oracle
Its hard for it to have enough impact to be much of a sideboard card unless were really valuing the 1/4 reach aspect (not just the enters-the-battlefield trigger)
If the green-white player runs low on good draws later in the game, the Invoker can pump anything into a serious threat, and it can fight with its 4/3 stats in a pinch