Would probably work though I don't know if you'd have enough damage for the duel.  Otherwise it's good on Rotos or Marius.

So, I tested this out with Goffred Brassclad, whose passive is similar - "Active Effect: If an ally is about to get killed by a fatal hit, blocks incoming damage and places a [Block Damage] buff on them for 1 turn. "

I'm guessing it should work the same way as that passive. The target doesn't die. In Goffred's case, they stay alive with full HP, and I don't think that it was because the target got oneshot. Also, the duel buff stays on the boss, and the debuff stays on the target.

In other words, to your first question, the answer is yes, to the second, the answer is ... maybe? I have no idea how you get enough damage to bring the Duel to an end. Guessing you need to use maxhp-based champs along with unkillable or block damage, but it's hard to fit that into a team, and especially hard in the case where you're also bringing someone useless like Goffred, who does basically nothing for the rest of the fight.

to be honest, no idea how much damage is required for it, but my Gnut can use his hit and live and Marius cud use his and it shud count now after 1 of Plariums fixes wud just need to revive and repeat