Can Glasc Mixology play a unit from his Deathknell to a battlefield where he is the attacker? Or just when he is the defender?

No, you cannot play the unit from your Deathknell ability to the battlefield you are attacking.

According to RiftJudge FAQs #9479 and #10612, 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 that battlefield during the combat cleanup phase (when the Deathknell trigger resolves), you do not have the permission to play units there.

This contrasts with a defending player: as the defender, you already control the battlefield where the combat is taking place, which grants you the permission to play the unit there to maintain your control (as established in FAQ #9149 and #10245).

Summary of Differences:

• If you are the Attacker: You cannot play the unit to the battlefield you are attacking because you do not yet control it.
• If you are the Defender: You can play the unit to the battlefield where the combat is occurring because you already control it, which helps you maintain that control.