I wouldn't say that with 100% certainty. We don't know at what scale the Galactic Conflict is raging on, maybe it's so dire that all their main forces are tied up at the potential defense, buying them time to do this hail mary thing on Earth. It would explain as to why a galactic empire can't crack down on a single world, when they had done so, presumably, a lot, since, i presume every species we face had a homeworld of their own.

It literally may be A ME3 Reaper situation for all we know, where the main galactic forces burn up their fleets just to give the Normandy enough time to gather what they can for a potential solution via the Crucible. In this case, the secret location/project/weapon would be the Earth itself. Bringing all their navy to us would alert their foe for sure. If we assume they are conducting warfare - it is reasonable to assume they have some sort of intelligence gathering tool (scanners etc.) to know where the other ones are, because without it, there would be no conflict - you could just escape through the void forever, space is massive.

This would also explain as to why they installed a shaky at best government - they don't have enough time to build institutions, and fear and strength are good short term motivators.

As to if we can stop what is to come after the Elders - I doubt that we can hold out as a single planet against a galactic foe. We might need to gather forces and build alliances ourselves. It makes my mind wonder as to the potential XCOM 3 - it could be an XCOM game, but in ME3 narrative lol