broken drain lever redux

Last month I asked for advice regarding retrieving the brass plug that (I assumed) was blocking the bathtub drain. I wasn't able to snag the plug--which continued to slip lower in the pipe, nor could I loosen the slip nut where the overflow pipe meets the tee pipe. A stronger friend visits about once a month, so I put the project on hold until the next visit. (Fortunately, we have 2 bathrooms.) Here's the update/new problem:

With the overflow pipe removed, it appeared that the plug had slipped a few more inches and now rests where the pipe makes an approx. 90 degree angle and runs between the joists. Further, judging by the feel against the rod, this does not appear to be the brass plug--the surface is irregular. It's impossible to look directly into the pipe (the access panel is partially blocked by a radiator) and pictures suffer due to the lighting. Judging by the reflection, it seems that whatever is blocking the pipe is white.

Before I discovered that there was nothing attached to the overflow lever, I assumed that the drain was clogged and I treated it with lye drain cleaner--several times over a few days. I've since learned that this cleaner can actually form hard crystal blocks in pipes when used in excess, which probably applies here.

Chipping at it with a chisel didn't do anything and it's too large/too firmly wedged in place to suck it out with a shop vac. (FWIW, all this effort has made a tiny difference. The drain now runs extremely slowly, as opposed to being entirely blocked.)

So, the new question--how can I dissolve/remove this large lye crystal?

Thanks for any advice/words of wisdom.

It may be quicker to just replace the tub drain.
They don't last forever anyway.

The Lye (sodium hydroxide) probably did crystallize on the stuck plunger.
I would first remove the radiator, if that is the only plumbing access, and remove the part of the drain where the plunger is stuck. Then, using gloves and goggles, try to remove the stuck plunger or just replace that part of the drain.