Jhin Keystone?

Warlord's is best since it gives attack speed (more ad for jhin) and movement speed (much needed on Jhin) plus if you go 100% crit you proc every 2 autos. but yes Deathfire is a good secondary option.

Either Warlord or Thunderlord. With warlord you get a heal and some bonus AD if you manage to hit your enemy with your 4th shot in the early game. Thunderlord can easily be procced with AA,Q,AA or even with your ult. If you proc Thunderlord with your last ult shot they're 99% dead if they're below half HP (a squishy of course).

Warlords, Some go the "metasheep" way by taking Thunderlords, but thats wasted potential

I'd say DFT or Warlords depending on the situation, if your support already heals I'd go for DFT. Warlords AD at level 4 is 5 more damage since it doesn't scale with AS items.