Frozen 3-Handle Shower Diverter Valve

Got a 40 year old Delta 3-handle setup in a fiberglass tub/shower. For the last 20+ years, the diverter valve has been left permanently in the "up/shower" position and has frozen/corroded that way. Can't turn it at all to run the water through the tub spigot.
Further problem is the slot head of the screw that holds the plastic handle on the diverter valve body started to strip when I tried to remove it .
Not sure how to proceed. Put a big pipe wrench on the diverter handle and try to force it? Grind the screw head off? Call a plumber :-(
There's no shut-off valve to the tub. And access to the pipes behind the shower is extremely difficult- up through the attic and then drop down behind the wall into a very narrow space that is filled eight feet deep with blown-in insulation!
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On 6/17/2019 10:48 AM, Oren wrote: ...

Vinegar probably do more good against the hard water deposits...or, never tried it for this, but the cola soda soak might be good...there is the commercial CLR for Calcium-Lime-Rust but I've had little luck with it. Lime Away has some hyrdrochloric acid.
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On 6/17/2019 11:33 AM, Wade Garrett wrote:

Can you put a shut-off someplace? At 40 years I'd consider just replacing it. Actually, after 40 years I'd think about just remodeling the bathroom and getting rid of the fiberglass. I did that a few years back.
You can try soaking it with penetrating oil or CLR and see if that help. It will only soak so far and if it is internal blocked with minerals will not do much.
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On Monday, June 17, 2019 at 11:33:52 AM UTC-4, Wade Garrett wrote:

First question is, do you really need to move the handle? How about installing a shower head fixture that includes a removable spray wand? That could be laid down and would fill the tub, though slowly.
If the backside is accessible from the attic after removing the insulation, all things considered, at least it is an option. Problem I see is that whatever you can spray on isn't likely to penetrate to where it's needed and it's likely frozen from calcium and the like. Maybe put some vinegar in via the shower head pipe, let it soak for a day or two? That might dissolve the minerals. Then put a pipe or pipe wrench on the handle and torque away. How lucky do you feel today?
Oh, and like Ed said, if there is a spot where shutoff to the bathroom can be added without too much trouble, might want to do that first. What kind of piping is this? Copper I hope?
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In alt.home.repair, on Mon, 17 Jun 2019 08:48:43 -0700, Oren

Tapping is very important. Every day at least. While you're soaking it in whatever, I think you can tap pretty hard, with something harder and heavier than rawhide, so hard the parts vibrate, which makes the liquid wrench etc seep farther in. Just not so hard you'll actually break parts.

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