Can attacking Glasc which gets killed during showdown summon units to the same battlefield it's attacking? (How about defending?)

No, an attacking Glasc Mixologist cannot summon a unit to the battlefield it is attacking if it dies during the showdown. However, a defending Glasc Mixologist is permitted to do so.

The Ruling

According to RiftJudge FAQ #10112 and FAQ #9479, when you attack into an opponent-controlled battlefield, you do not control that battlefield until the showdown is resolved and you have successfully conquered it. Because you lack control of the battlefield during the combat cleanup phase (where the Deathknell trigger resolves), you are not permitted to play a unit to that location.

Conversely, a defending player already controls the battlefield where the combat is taking place. Therefore, they are permitted to play a unit there via Deathknell even while the combat is ongoing (consistent with RiftJudge FAQ #9541).

Sequence of Events (Attacking)

1. Combat Damage: You attack with Glasc Mixologist into an opponent-controlled battlefield. The unit takes lethal damage and dies.
2. Deathknell Trigger: The Deathknell ability triggers and is placed on the chain as a pending item.
3. Resolution Attempt: 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 still in the process of attacking to conquer it), you are not permitted to place a unit there.

Sequence of Events (Defending)

1. Combat Damage: You defend with Glasc Mixologist at a battlefield you control. The unit takes lethal damage and dies.
2. Deathknell Trigger: The Deathknell ability triggers and is placed on the chain as a pending item.
3. Chain Resolution: The game pauses to resolve the chain. You resolve the Deathknell effect and play a unit from your trash to the battlefield.
4. Control Check: Because you already controlled the battlefield when the combat began, and the Deathknell resolution happens before the final control/conquest checks, you are allowed to play the unit to that location.