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]

The same result applies when one kill instruction kills multiple units. Their deaths are simultaneous because they result from the same game action,^[370.1.a.2] and each relevant Deathknell becomes pending before its unit moves to the trash.^{[428.1.a.1.b][808.1.d.2]}

In either case, all units that died from that event are in the trash before the pending ability is finalized and its target is chosen.^{[334.2][355.8]}

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,^[190.4.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,^{[466.1.a.1][334]} so by the time Glasc's Deathknell finalizes and the new unit reaches the battlefield, the enemy is already back to full health. Because Glasc was your only defender, no defenders remain during cleanup step 3d, so the surviving attackers are not recalled.^[466.1.a.2]

With units from both players now present, combat result is determined. Because both players have units there, the result is "No Result."^[466.3.d] A fresh combat is immediately staged at that battlefield.^[466.3.d.1]