"then why didn't he compel Astral Plane/Avernus Tav at the party?"
It's very easy to explain. (warning: discussion of abuse)
Cazador can't compel his spawns when they don't hear him. Confirmed by Spawn's epilogue when Cazador is still alive - Spawn is still free, he wasn't forced to come back through distance. It's also confirmed in case gurs help him to kill Cazador behind the screen: Spawn says they ripped off his tongue, after that he was able to approach him and kill him. Leon and Aurelia weren't forced suddenly, their compulsion only turned on once they tried to go directly against Cazador's previous order - to capture Astarion. This is the reason AA couldn't force Tav to get back from Avernus or Astral Plane - because they couldn't hear him. He didn't force Tav not to go with Karlach/Laezel, because all the other companions would have killed him if he tried to control Tav in front of everybody. If he tried to begin to compel Tav at the party, again, other companions could have turned against him, being warned previously by Tav. It's very different than being already told not to tell anyone before the party. So in my opinion, AA is just lying when he says he could have said a word - and Tav would have magically returned to him from Avernus. He is trying to gaslight Tav and keep his face. Which only proves further, how he constantly reinforces Tav's slave status to their face.
