To be honest I dunno what is up with everyone being from Arcadia. :)

The only thing I am not sure about is this second battle of Arcadia mentioned in Björkgård history. There wasn't really a second battle. After the first battle UNSC presence on the world disappeared and a vast majority of the population abandoned the world. All that remained was a few farming communities that continued to supply the UNSC. By the time of the glassing of Arcadia the world was pretty lawless and lacked permanent UNSC fleets or land forces. The glassing was pretty much a massacre.

That being said, there really isn't much canonical information on the fall. So there isn't anything to say that there wasn't some form of UNSC resistance.