Does the defender still have control of the battlefield when their deathknell triggers are on the chain, and can they react with hidden units (like Zhonya + Faefolk) despite not having anything else on that battlefield?

Ruling: Yes, the defender maintains control of the battlefield when their deathknell triggers are on the chain because losing contested status is not part of cleanup. This means hidden units like Zhonya can return to base and stage another combat.

Sequence:

• Deathknell triggers go on the chain
• Contested status is maintained (losing contested status is not part of cleanup)
• Defender retains control of the battlefield
• Hidden units can be played to that battlefield
• Playing the unit to the battlefield stages combat again
• You cannot react until cleanup (and any subsequent cleanups) are done, which is just after 444.1 and subrules complete and before 444.2

Nuances:

• If the battlefield is NOT contested (e.g., picked off with Falling Star), you immediately lose control and cannot react with hidden cards at that battlefield (they're trashed before the deathknell is finalized)
• You can still react with cards from hand that have the reaction keyword even if you lose control
• The lapse in presence of units would unstage combat, but it gets staged again when you play the unit to the battlefield