Chain stuck between cassette and spokes and damaged rear hub - any way to fix it?
That's a bit of a mess. Looks like the chain dinged up the hub, freehub body (that the cassette mounts on) and the spokes.

The freehub body is removable, and you could possibly use a fine file to remove any metal burrs that are keeping the freehub from rotating. 

What is far more worrying is the state of the spokes. All the drive side spokes are gouged and at least one is bent. I'd have a bike repair shop look at it to assess the extent of the damage. Best case you can replace a few spokes, worst case all the drive side spokes need replacing, or if the hub is not salvageable you are looking at a new wheel.

Have the bike repair shop adjust your derailleur while you are there.