Let's turn things around a little as a thought exercise. Let's say returner's partner raised a finger to indicate a long serve, and that both server and server's partner saw it, but pretended like they didn't see it. Receiver returned the ball as a continuation of her motion although she did see the ball out (but didn't call it herself since she has agreed with her partner that receiver's partner should call the service line).
Server continues the act by hitting the next shot, so that her partner can stop the point and claim a let and a first serve. Seeing the server continue the point, returner would have been perplexed leading to the question of whether the ball was in or not (asked out of confusion regarding the situation, not because returner has any actual doubt).
Of course I'm not suggesting this is what happened. But it illustrates why an automatic let and 1st serve is not mandated. If it were, server and her partner could use such a tactic whenever a 1st serve is called out but still returned into play (which happens fairly often). Pretend you didn't hear/see the call, then after the ensuing confusion claim a let and a 1st serve.
So: Either the out call was made, or it was not. If it was made, 2nd serve. If it was not made, point to server (based on hindrance). And you have to go by the word of the returners.
Now, it would be nice of returners to offer a 1st served based on the confusion of the situation. But not by rule.
If it can be determined that an out call was in fact made, then it's a second serve. It would only be a first serve if the returners felt like the server was delayed significantly between their first and second serves.
It cannot be a hindrance because any hindrance call must be made in a timely manner and before any attempt on the ball is made. It would not be in the best interest of the game to decide which hindrances cancel out based on order of occurrence. Since multiple people on different sides of the net were talking and play continued on for several shots, any claim of hindrance should be thrown out.