Can I react to Lonely Poro Deathknell trigger with an Ambush unit to send it to the battlefield and still draw the card from the Poro?

Yes, you can play an Ambush unit while a Lonely Poro's Deathknell trigger is on the chain, and you will still draw the card.

The Ruling

You will draw the card from Lonely Poro's Deathknell because the "alone" condition is checked at the moment of death, not when the trigger resolves. Since the Lonely Poro met the criteria of being "alone" when it died, its Deathknell trigger is valid and will resolve successfully, regardless of whether you play a new unit to the battlefield later in the chain.

Why this works

1. "Alone" is a Historical Check: According to FAQ #373 and Rule 808.1.d.3, Lonely Poro's "alone" condition is satisfied if there were no other friendly units at that location at the moment of death. Once the Deathknell trigger is created and added to the chain, it retains the information that the condition was met.
2. Chain Resolution (LIFO): Per Rule 326.1 and Rule 359.3, the chain resolves in Last-In, First-Out order. Even if you play an Ambush unit as a reaction to the Deathknell trigger, the unit enters the battlefield before the Deathknell trigger resolves. Because the "alone" condition was already locked in at the moment of death, the arrival of this new unit does not retroactively invalidate the Deathknell.

Sequence of Resolution

1. Death Event: Lonely Poro dies while "alone" (no other friendly units at its location). The Deathknell trigger is created and added to the chain.
2. Reaction: You play an Ambush unit in response to the Deathknell trigger, adding it to the chain.
3. Resolution: The chain resolves (LIFO).
   • First, the Ambush unit enters the battlefield.
   • Second, the Lonely Poro's Deathknell trigger resolves. Because the condition was satisfied at the time of death, you draw your card.

Summary: Your Ambush unit arriving at the battlefield does not invalidate the Deathknell because the "alone" check is based on the state of the board at the time of the Lonely Poro's death, not the state of the board when the trigger resolves.

Note: I could not fully verify this ruling against the retrieved rules.