When it enters, you can tap X creatures and put a stun counter on any of those creatures you dont control, which means they dont untap for a turn
Shield counters don't prevent players from sacrificing permanents
Seaside Citadel ($2) Yes, all four of them are on the list of things you should stock up on
Yorion does make you play 80 cards instead of 60, but for decks not reliant on drawing specific cards early in order to win, there is enough redundancy in Magics rich card pool to make Yorions caveat not too big a deal
Krang, the All-Powerful amplifies your draw payoffs, and we cant forget about alternate wincons like Laboratory Maniac if you include some self-mill with your extra card draw
Improvise can never pay for any colored mana in a spells cost, nor for specifically colorless mana (like the final mana symbol in Reality Smashers mana cost)