If I have an Ancient Pokemon in the Active Spot that attacked last turn and I use Thorton to switch it for a Koraidon with "Unrelenting Onslaught" attack, would I get the bonus damage for "Unrelenting Onslaught"?
No, all effects carry over when using Thorton, so the game sees Koraidon as the same Pokemon for all intents and purposes, including having attacked last turn.
Source: TPCi Rules Team (2024-12-19)
If I use Koraidon's "Unrelenting Onslaught" attack, can I get the extra bonus damage if my different Ancient Pokemon attacked while confused and flipped tails?
No. A failed attack attempt while Confused does not count as an attack.
Source: TPCi Rules Team (2024-11-14)
If I use Koraidon's "Unrelenting Onslaught" attack, can I get the extra bonus damage if I attacked last turn with a different Koraidon?
Yes. Unrelenting Onslaught only cares about whether a different Ancient Pokemon attacked during your last turn.
Source: Surging Sparks FAQ; TPCi Rules Team (2024-11-07)