The fact that having an enchantment and an artifact in play means that you can use both abilities is incredible and makes it even better in draft
That said, if your deck does run Equipment, +1/+1 counter effects, or anthems, Esper Sentinels tax can get truly untenable, virtually guaranteeing you three or more cards per turn cycle
Yes, undying is a triggered ability, and triggered abilities use the stack
Target creature gains hexproof and indestructible until end of turn
We'll also add one more copy of Benja - Guardian of the Dragon Gem as item decks are prevalent in the meta, so having more ways to deal with them is good
If youre just going to win via Scute Swarm, build up as many as you can, and cast Biomass Mutation to make them as big as you want