Has the cold war started back up or did it never really end?
Yes and no. The " cold war" is indeed over, but the "cold war" should really be understood more as an single military engagement, not as a entire war. The real war is one of global dominance. Each side erroneously believing that their way is the best and is to become the ultimate government on the face of the globe.
Is this as sugested by the Russians a change in US poliecy?
It is if this is what it appears to be, which I doubt. The real question is to what end have these arrests been made. It is common to allow sleeper cells to operate inside the U.S. A known sleeper, or even an active spy cell is a benefit to the U.S. Control can be exercised as to what information they get access to. The future plans of the opponent can be determined by the information the cells attempt to gather. To alter the U.S. response to Russian spying ( or the Russian response to U.S. spying ) significantly alters the game.
Is it possible that the Russians were spying on some other country's behalf?
Highly unlikely. The Russians have always known that knowledge is power. They have and will continue to keep gathered information under their control and for their own use. A more likely scenario would be a spy handler gone freelance to a third party. Neither government would appreciate that. The current drama playing for our entertainment looks very much like that to me. We "capture" the spies, The Russians are shocked. Diplomatic measures are taken and peace assured. Meanwhile the handler relocates to someplace with permafrost, and the U.S. gets to convict some spies for a public show ( under the blanket of national security so no one sees the sleight of hand as they get sent home, or enter the protection program for defectors. )
The above is only my own hasty, uninformed, opinion of course.
But it is free and worth exactly what you payed for it. LOL
Cheers,
Twisted