If the lockring has come off as theorized, all you'll need to do is take the wheel off and all the cogs should come freely off the hub.

I'm suspicious of this diagnosis. For both cogs 1 and 2 to spin, both they and the lockring would need to move outside of the end of the freehub. There's rarely that much room.

I suspect a busted freehub (that's the part the cassette cogs fit onto), or badly mutilated cog splines. Interested to hear the diagnosis.