Well, it was also stated that Earth's Well is the only well that granted its power to the denizens of its planet, making it a much more "lucrative" target, but also a much more potentially harder-to-take-over target as well. So it stands to reason that the Battalion would want to understand our Well, and undermine its unusual defenders, before beginning a full assault.
Harder, maybe. But to a race that devoured uncountless Wells in the past, and have Prometheus *and* the Menders scared shitless? The guys who already know exactly how the Battalion will try to beat us? We're a race of beings that was almost wiped out by an army of alien pacifists that didn't even have access to magic powers.
When I conceptualized the Immortal Game backstory, I concluded the only thing that could possibly be a threat to something like the Battalion was an Ascended being. I ended up piling on Rularuu, the embodiment of Earth's Well, *and* Hamidon to make it credible. Trying to write it as the Battalion has thousands of Wells of power, but we have the really good one so its a tie, is, well, I can only describe that as cheap.
The other really weird problem with the sleeper agent idea is that undermining Vanguard would weaken Earth but Lady Grey helped form Vanguard in the first place. Why would you make a defense force you were going to then undermine? Wouldn't it have made more sense to join the Midnight Squad, the Freedom Phalanx, the Salvation Army, something that already existed that you could undermine? If you make it, then take it away, that's a net zero. 99% of the combat forces that would attempt to repel a Battalion invasion of Earth aren't Vanguard. It would even have made more sense for Lady Grey to join Arachnos, because that's a credible fighting force that would be a potential threat to the Battalion's invasion forces.
Since, as you say, Earth's Well is taking matters into its own hands by just superpowering everything in sight as a defense against the Coming Storm, disrupting Vanguard seems to be a relatively trivial endeavor.
Incidentally, if the Battalion have been here all along and were observing us looking for the right time to strike, that time was December 2002, after the Omega Team closed the Rikti portal and after enough time had elapsed for the remnants of the Alpha team to disband. There has never been, and would never be, a better time to strike Earth than then. We could have been taken over by the Mystery Men.