If a Stellacorn that has had two disciplines cast on it and Glowstone triggers at the end of turn, dealing 5 damage to everything, does Stellacorn die or live?

Stellacorn lives.

Two Disciplines give it +4 Might this turn, putting it at 7 Might (3 base + 4). Glowstone's end-of-turn trigger deals only 5 damage to units you control, which is below Stellacorn's 7 Might — so it isn't lethal (Rule 142.4). The trigger resolves during the Ending Step while the "this turn" buffs are still active, and even if it had been damaged, the ending cleanup heals all units anyway (Rule 317.2).