If a Karthus dead with temporary and a Soaring Scout die with temporary, what passes?

If Karthus, Eternal dies (whether from Temporary or any other cause) at the same time as another unit with Deathknell, his passive still applies — that unit's Deathknell triggers an additional time.

This is because Karthus's ability ("Your [Deathknell] effects trigger an additional time") is a passive ability, not a triggered one. Per FAQs #10742, #9317, and #9711, passive abilities remain active at the exact moment of death — they don't need to "survive" to evaluate their trigger condition. So when the simultaneous-death event creates Deathknell triggers (Rule 808.1.d.2), Karthus is still on the board and his passive doubles those triggers.

If instead you return Karthus to hand before the Temporary kill resolves (responding to the Temporary trigger while it's on the chain), his passive is no longer active on the board when Deathknell triggers process — so no bonus in that case (FAQ #10779).

I noticed multiple unrelated questions. I've answered the first one — please ask the others in separate messages so I can give each one proper attention.