My lonely poro died alone to the opponent's attack. Its deathknell is added to the chain. Can I react to my lonely poro's deathknell and play nidalee to the lonely poro's battlefield in my opponent's turn? Does this mean that another combat starts and I still draw 1 from the lonely poro's deathknell?

No to the Nidalee play, but yes to the draw — the problem isn't timing, it's location.

Reaction does let you play her during a Closed State on your opponent's turn, and you still control that battlefield mid-combat (control isn't lost while a chain or Combat is there). But Nidalee's Ambush is only permission to play her "to a battlefield where you have units" (Rule 822.1.b), and since your Poro is already dead and in the trash when the Deathknell trigger is on the chain, that battlefield has no units of yours — Rule 822.3 makes it an invalid destination. You can play her to your Base, or to another battlefield you control that does have your units.

The draw is unaffected either way: the "alone" check is historical, locked in at the moment of death (Rule 808.1.d.3), so Lonely Poro's Deathknell resolves and you draw 1 even if a unit shows up later in the chain.

And no, you don't start another Combat by playing a unit — that Deathknell window is inside the Combat Cleanup of the same Combat, and a unit arriving mid-combat just gains Attacker/Defender designation in the cleanup after the action that brought it (Rule 459.2.b.3.a). Combat only stages when opposing units sit at a battlefield in a cleanup with no Combat already in progress (Rule 461).