the Phantom

the Phantom's

Blackquill would not want Phoenix as his attorney, because Phoenix was known to find the real culprit and he was protecting Athena.

I don't think Lana wanted Phoenix as her attorney since she was protecting Ema, but it still happened. Aura, who definitely wanted the truth and gave no shits about Athena (especially since she suspected Athena of being the culprit) would have probably gone to Phoenix and requested his services for her brother. I don't know whether or not Simon would have accepted this, but I'm sure Phoenix could have convinced him. If Athena herself somehow wound up meeting Phoenix while he and Aura were trying to convince Blackquill to accept Phoenix as his lawyer, I think Athena would definitely help convince Simon. Edgeworth might not have been Chief Prosecutor at the time so he likely wouldn't have been concerned about Blackquill as a subordinate, but Blackquill's case is very similar to his and Lana's cases combined. I think that would have piqued his interest and concern and he might've been inclined to get Phoenix on the case.

In the game, Blackquill had a pretty strong will to keep the "truth" a secret, but here, it's still fresh. He doesn't have 7 years of prison to solidify that will. He would be easier to convince then, compared to the Blackquill we saw in Dual Destinies.

Aura may still have tried to force the trial through kidnapping, and Phoenix may have just happened to have been free at the time. I don't doubt Edgeworth wouldn't have become Chief Prosecutor because Phoenix didn't have a badge or whatever, so it's possible that Edgeworth might've asked Phoenix to help him with the trial.