The drain mechanism in my bathbut is the falling plunger type, and is operated by a little lever at the foot of the tub. Flipping the lever up allows the plunger to drop and seal the tub drain. Flipping the lever down lifts the plunger and allows the tub to drain.

Problem is, the plunger is frozen in the raised position. I've spent hours with various tools and mixtures in an attempt to soften the corrosion so I can get the plunger and linkage out via the lever hole. For my troubles, the 2 linked rods came unlinked, and the plunger never did budge.

I've tried jamming a washrag soaked in vinegar down there and letting it soak overnight. I've tried a bottle of CLR to no avail. Is there some kind of gripper or notched device attached to a flexible shaft that I might snake down there and twist the plunger free?

Or any other comments?

The only alternative I can see is to break the drywall on the other side and disassemble the entire drain stop mechanism.

 

It would be easier to attack this from below if you can.
Take p trap apart and use a long metal rod or long screw driver to loosen it up.

Take your time, do not get frustrated because you may cause even more damage to the drain.