Can followers (templar, enchantress, scoundrel) die?
No, they don't die. If they go to 0 health, they disappear from the battle and reappear sometime later (even if the same fight is still going on).
It was easiest to test this with the skeleton king. My templar "died" a couple of times and came back 30 seconds or something like that later (Although that number is just an estimate, I haven't measured).