If i play leblanc and play glasc mixologist and my battlefield was cleared, can i use glasc mixologist deathknell while on chain to place units revived into the same battlefield? is the battlefield still mines while on chain. what if there are opponents?

Yes — you can play the unit from Glasc Mixologist's Deathknell back to that same battlefield, and yes, it's still yours while the trigger sits on the chain.

Control of a battlefield can't be lost while there are items on the chain (Rule 187.4.c); the cleanup control-loss check only runs in an Open State (Rule 323.6). Mixologist's Deathknell is added as a Pending Item before the unit moves to the trash (Rule 323.4 / 808.1.d.2), so when it resolves you still control the battlefield and it's a legal destination for the unit you play from trash.

If opponents still have units there, the result is the same: the battlefield is Contested and a Showdown/Combat gets staged, but you remain its controller until that resolves, so you can still play your Deathknell unit there — the "can't play into a battlefield you're attacking" restriction only hits the attacker, not the controller (FAQ #9149). The revived unit will be present for the upcoming Showdown/Combat, though if Mixologist died to combat damage, enemy units will already have healed before your Deathknell resolves.