The triggered ability triggers once for each creature blocking or blocked by Assembled Alphas
Red often requires you to discard as part of the bargain, sometimes giving you card "selection" but not actually card "advantage." Black is the most likely to cost you life and White is just the worst, often having to rely on shutting down an opponent's extra draw in order to get your precious extra cards
There are lots of cards that can make Universal Automaton cost
Unmarked Grave A powered-down Entomb, Unmarked Grave is going to do much of the same things Entomb does, for a nonlegendary creature, slightly more expensively, and, more critically, as a sorcery
Do keep in mind, though, that they are not the same character