Mixologist targeting unit at same battlefield

Yes — you can play the unit from Glasc Mixologist's Deathknell to the same battlefield where Mixologist died, as long as you control that battlefield (i.e., you were the Defender there). Control of a battlefield can't be lost while there are items on the chain (Rule 187.4.c), and the Deathknell trigger is a pending item, so the battlefield stays a legal destination for the play.

If you were the Attacker, no — you don't control the battlefield until you conquer it, so you can't play a unit there during the combat cleanup.

Two notes on the wording: the ability doesn't "target" a unit — you choose which unit (cost 3 or less, power 1 or less) as the Deathknell ability resolves, so nothing gets chosen at finalization. And the revived unit arrives after Combat Cleanup, so any enemy units there have already been healed; the new unit can't deal damage in that damage step — it just counts for the combat result/control check.