my lonely poro is attacking alone to an enemy's battlefield. It dies and deathknell is added to the chain. Since the lonely poro's deathknell is still on the chain, i am still the attacker and my lonely poro hasn't gone to the trash yet. Can I play my nidalee to that battlefield in reaction to my lonely poro's deathknell?

No — you can't play Nidalee there.

Ambush only lets you play her to a battlefield where you control units (Rule 822.1.b), and your Poro is already dead and off the board when its Deathknell goes on the chain, so you have no units there. Being the attacker doesn't help: attacking grants the Attacker designation, not control of the battlefield — you only gain control after Conquer resolves (Rule 469.1.b), and Reaction/Ambush placement still follows normal restriction of your Base or a battlefield you control.

Deathknell being on the chain doesn't keep the Poro "present" — the kill already happened and the trigger just remembers its state (Rule 808.1.d.3). You could play Nidalee to your Base or another battlefield you control during that window, just not the enemy's. This matches FAQ #11429 exactly.