What happens when I react to a Deathknell trigger of one of my units in a combat with Janna? Will all units be healed?

No, playing Janna in reaction to a Deathknell trigger after combat will not result in your units being healed by her — they've already been healed by Combat Cleanup.

Here's the sequence per the FAQs (#9514, #10157):

1. Combat damage is dealt.
2. Units with lethal damage trigger Deathknell (added as pending items).
3. Those units are killed (moved to trash).
4. Combat Cleanup heals all units in play — every unit on the board, regardless of location.
5. Only after that does the game return to FEPR, where the pending Deathknell triggers are placed on the chain and players get priority to respond.

So by the time you have a window to play Janna as a Reaction to the Deathknell trigger, the Combat Cleanup healing has already happened. Janna would heal "your units here" at her location, but they're already at full health from step 4. The heal is redundant — not because Janna doesn't work, but because there's nothing left to heal.

One nuance: If the Deathknell trigger itself deals damage (some do), then you could use Janna after that damage resolves to heal your units at that location. But the initial Combat Cleanup healing happens before any Deathknell triggers resolve.