Glasc Mixologist

Yes, because targets are not chosen until the ability is finalized — by which point all units that died in the same event are already in the trash.

Whenever a unit with Deathknell dies, its trigger is placed on the chain as a pending item.^[808.1.d.2] During a cleanup, this ordering is enforced system-wide: step 3a queues all Deathknell triggers from every unit with lethal damage^[323.4] and only then does step 3b move those units to the trash.^[323.5] Finalization of pending items does not begin until the cleanup task has run to completion.

Yes, as long as you controlled that battlefield when Glasc Mixologist died.

Outside a showdown, the Deathknell trigger remains on the chain as a pending item from the moment it is queued. Because a chain exists, the turn remains in a closed state,^[309.1] and cleanup step 4 — which makes unoccupied battlefields uncontrolled — only applies during an open state.^[323.6] The battlefield stays under your control, so when the unit is finalized you can choose it as a valid destination.^[355.2.a]

During a showdown, control of the battlefield cannot change until instructed by the steps of the combat or showdown,^[187.4.b] so the question does not arise.

Several things here commonly catch players off guard: surviving enemy units heal to full before the new unit arrives, neither player wins the original combat, and a second combat immediately follows.

The combat cleanup heals all units before any pending trigger can resolve,^{[461.1.a.1][335]} so by the time Glasc's Deathknell finalizes and the new unit reaches the battlefield, the enemy is already back to full health.

With units from both players now present, combat result is determined. Because both players have units there, the result is "No Result."^[461.3.d] A fresh combat is immediately staged at that battlefield.^[461.3.d.1]