so what do we think their chances of going to America by way of the moon are?

Geisterdamen city (Citadel of Silver Light) could be on the moon. Would explain the spiders. Low-gravity spiders? Yeah, they'd get huge and have those long spindly legs. And also why no one's found the place yet - only accessible by Queen's Mirror? See also: is on the moon.

(I'm assuming either a native moon atmosphere or pressurized caverns, I guess.)