Even though most Doomsday lists dont play Cavern of Souls in the main-deck at the moment, you should always try to counter Doomsday instead of letting them make a pile
To proliferate, you can choose any permanent that has a counter, including ones controlled by opponents
mana creatures can be victims of removal spells, and artifacts that ramp can be answered by many different spells, but land destruction is a rare option
First, the active player (or, if it's not an opponent's turn, the next opponent in turn order) chooses which creature they'll exile, then each other opponent in turn order does the same, knowing choices made before their choice
Double on-hit triggers is good, optional removal is worth the mana cost
But I'm glad they did since Swarmyard Massacre is the best black board sweeper we've seen in years