If I attempt to conquer with Glasc Mixologist in an empty battlefield and my opponent Hidden Blades him, can I play my Deathknell unit in the battlefield I was attempting to conquer?
No, you cannot play the unit from your trash to that battlefield.
The Ruling
According to FAQ #9184, when you move a unit to an empty battlefield to attempt to conquer it, you are the attacker and you do not control that battlefield until the showdown is resolved and you have successfully conquered it. Because you do not control the battlefield, you cannot play units there.
Sequence of Events
- Move: You move your Glasc Mixologist to an empty battlefield. This opens a non-combat showdown.
- Hidden Blade: Your opponent plays Hidden Blade (as a [Reaction] from Hidden or an [Action] if they have priority) to kill your Glasc Mixologist.
- Deathknell Trigger: When Glasc Mixologist is killed, its Deathknell ability triggers and is added to the chain as a Pending Item (Rule 734.1.d.2).
- Resolution Attempt: When you attempt to resolve the Deathknell ability, you are instructed to play a unit from your trash. Because you do not control the battlefield you are currently in, you cannot play a unit there.
Why this differs from the Glasc Mixologist FAQs
The FAQs you referenced (such as FAQ #9149 and FAQ #8852) involve a defending player. A defender already controls the battlefield where the combat is taking place, which is why they are permitted to play units there via Deathknell. As the attacker moving into an empty battlefield, you have not yet established control, and therefore lack the permission to play units to that location.
Note: I could not fully verify the specific interaction regarding playing the unit to your base instead of the battlefield against the retrieved rules.