I often get the threads freed up but the line is fused to the nut and spins.

If they don't come loose its not worth fighting too much IMO. Nip the lines off with a bolt cutter and bend up new when it is time to put it back together. Easier to get the nut off on the bench in case you are saving the wheel cylinders or calipers. Most of the time they need rebuilt or replaced anyhow.