I know about the recording in the locust stronghold , but I never got the impression that sinking Jacenta and flooding the hollows was intended as a means to stop the lambent. Since it's mentioned that the locusts already realize that sinking the city would flood the hollow so most likely they had planned to evacuate before that happened. Although since it was Adam talking about the plan to sink the city , the idea that sinking the city was meant to stop the lambent does make sense.
The part about the COG rejecting Adams plan was implied I believe when you get back to Jacenta and speak to the chairman. Without going back and double checking , I think he mentions something like "So he was right after all". Which would mean that most likely Adam had suggested the plan before , but I could be wrong about that.
As for the dialogue with the queen and the comm link with Adam after the credits , I don't doubt that he had some dealings with the locusts and maybe even was an ally with them. But for what means? Was he a traitor to his people? I get the impression that he was trying to make a pact with the locusts , knowing that perhaps the lambent would become a far greater threat. Maybe get the humans and locusts to align against them?
Only other thought that comes to mind is that there is something about the immulsion. Something that maybe Adam , the Queen and maybe even the Chairman knows about.