If I attack with a Lonely Poro and it dies in a 2 might to 2 might trade, can I react to the Poro Deathknell to play Nidalee to that battlefield before it dies? (if it was enemy controlled)

No, you can't play Nidalee there.

Even though Deathknell triggers go on the chain (creating a window to play Reaction cards), Nidalee still follows the normal unit placement rules. Rule 813.3.a is explicit: units with Reaction can only be played to your Base or a battlefield you control. Your attacking Poro was at an enemy-controlled battlefield — you don't control it, so Nidalee can't go there.

Two other things block it too:

• Nidalee's Ambush says "to a battlefield where you have units" — your Poro just died, so you have no units there anymore.
• The attacking player doesn't gain control until after Conquer resolves post-Combat (Rule 464.1.b). Mid-combat, that battlefield isn't yours.

You could play Nidalee to your Base or another battlefield you control while the Poro's Deathknell is on the chain, but not to the enemy's battlefield.