The idea behind moving to Fort Dragonia was that it was a better place to defend against Porre than the Manor. It's likely the plan was to lure them in, or that Porre on its own would prioritize going after them first and then go after Termina/Guldove/etc. Of course, thanks to Serge, Porre couldn't even take the bait even if they wanted.
Lynx told Viper about the Frozen Flame. It's possible, as Radius especulates, that the General may want to use it to stop Porre's invasion or rule the world himself. Regardless for what, now that he knows about it, Viper wants it, and thus is willing to hear Lynx out. So it's likely Lynx mentioned Serge was the only one who could open up the way, but without giving exact details on the why, most likely. If he ever questioned it, most likely it was offscreen, and briefly.
Yeah, no clue about Lynx's speech in Viper Manor and Viper just rolling with it, though.
