Theory: Nigel Decided to Join the Choten Before the Events of “Heavenly Creatures”
Since Master Nigel is one of my favourite duel masters, I watched "Heavenly Creatures" several times. And while the reveal that Master Nigel joined the Choten happened at the end of the episode, I think that the agreement actually happened before.
In the episode, everyone is looking for the Light artifact in San Campion. However, as the artifacts were typically initially found in their home civilization, I don't see why they would be looking in the human realm while ignoring the creature realm. However, if Nigel either convinced the duel masters that the artifact wasn't in the light civilization, or he has other people looking there/he will look there, that could explain why everyone is looking elsewhere for the artifact.
The second reason I think that Nigel joined the Choten is that he went with Ray, Gabe, and Allie to save Sasha. When I first saw this, I figured the creators/writers simply wanted an episode where Nigel is with the main characters in the Light civilization, but there may be more to it. He may have figured that by having the kids there, they could also help find the Light artifact. As he had their trust at the time, they would have no reason to avoid giving him the artifact. This is further suggested by how Nigel jumped through the veil opening he made before checking if the acolytes are coming. Considering he didn't seem to care too much about going to the Light Civilization before, I doubt that he would have went to save Sasha on his own. However, if he planned to go there anyway, and to use saving Sasha as an excuse, then him being on his own would be fine.
One reason I wondered if Nigel joined the Choten at the end of the episode is because he didn't do anything to hurt the main characters, and even helped Sasha escape. However, this is easily explained. Even after the reveal, Nigel still pretended to work with the Duel Masters for a bit. This implies that he was undercover, so hurting the acolytes would ruin his plan. By pretending to be on the acolyte's side, he is keeping his appearance of a loyal Duel Master. He didn't need to betray the acolytes at the time because the Heart of Light would still be close to him, making it possible to steal it, or tell the Choten where it is, when the time is right.
So, as much as I like that episode, now I will be thinking about how Nigel was manipulating the acolytes the entire time.