Re:
Have you tried cleaning the internals, I have had many that looks like a total lost cause, all have come good after a good clean.
The problem is the old Shimano grease, it goes hard and the lever seem broken. Fires through, no ratchet, etc etc.
Remove the shifter from the brake lever and the bottom cover, then soak it overnight in white spirit, try to operate it occasionally when soaking. I bed you find that it now works. Shake the white spirit out and lube with something like GT85.