Do failed escort quests permanently kill the NPC?
Solution 1:
Yes, unless you have a Wakestone to revive the NPC when they are dead.
Solution 2:
No. It's unclear to me which NPCs respawn and when, but I can personally attest that at least Asalam the innkeeper respawns. There are claims that it takes an ingame week or that he reappears in the end game, but these are flat out wrong.
I had Asalam appear after more than an ingame week before I had even met the Duke.