If some of the targets of Captain America's first ability become illegal, the original division of damage still applies, but the damage that would have been dealt to illegal targets isn't dealt at all
Being able to cast one spell and get another for free helps counteract a few of the problems with this deck, notably its somewhat lacking land base for such big spells, and the deal is made even sweeter with some of the enters-the-battlefield effects you could be unexpectedly triggering
However, the roles cantrips play in certain decks varies according to the archetype using them
As long as your devotion to blue is less than five, Thassa isn't a creature
Out with fetch lands, in with sorcery WUBRG
At the height of The Kami War, O-Kagachi himself entered into the mortal realm in search of the part of him that had been torn away