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Aside from replacement being a decent option and probably achievable since the rear panels are usually flimsy.
They often come off with considerably more force than you want to apply without knowing for sure that you've undone anything that's holding them on. If you can identify them and find the plan and check that grub screw is the only thing holding them on and that's undone, you should be able to tap them and wrench them a bit to try and free them up.
It can get worrying before it suddenly lets go.
My kitchen tap is dripping. I plan to change the valves but I need to take the thing apart to check how many splines are on mine. Only I can't get the damn thing apart. I've removed the grub screw from underneath but I can't get it to budge. Any ideas for pulling the handles off?