"It's not just a matter of magic, but what sort of magic. There are hundreds of magical beings across their world, after all, in LA and other places. Twilight can obviously pinpoint which signals are the Slayers.

Even if that wasn't the case, there's no indication that even Willow could generate enough magic equal to the amount of a whole Slayer army."

She wouldn't have to, if she could find a way to cross Twilight's signals.

"It was last resort, and they did make it as heavily fortified as was within their abilities. They don't have all that Chinese military artillery just for kicks."

They're still on a wide open field with little cover. That's an easy no-no.

"Their last known location was somewhere in the middle of the ocean, not somewhere in the mountains of Tibet."

No, their last known location is whereever Twilight's magic stops detecting them. And that's Tibet.