There are occasions where it is good practice to retain older things...sometimes, it is just impossible or impractical. I think you may have reached the impractical and maybe the impossible stage - it's probably time for a new valve. While sometimes, you can get parts long after a thing has stopped being manufacturered, but that depends on how many were built, whether the company is still in business, and whether it is of good enough quality to justify fixing rather than throwing it away.
Some of the stuff was junk when it was built. Some designs are much easier to repair than others, and some are not really designed with maintenance or repair in mind.
I think there's a big difference between lack of competence and lack of knowledge. That's why I come here. The broken diverter means I need to replace the whole valve body, and the acid water is what weakened & eventually fractured the diverter tube. No incompetence there.
What is the significance of the distance from the centerline of the valve body to the place on each tap where the threads for the bezel begin?
My current Elger is about 2 1/2" long. The new production stuff I have seen has a much shorter distance. About an inch shorter. I don't know where to look in the catalogs for specifying that distance, to get a replacement with the same length.
If the replacement is shorter by an inch then it seems that I need to also buy a bezel specially for that length, which will project backwards into the wall to reach the threads? Seems like buying just the valve body is not going to work out; might as well get the whole kit even though I don't need the showerhead or spout.
And how do you reach so far back to remove a ceramic stem? Special tool like a deep socket? I want the tool for whatever brand/style I end up getting.
There are deep plumbing sockets, but if you have a deep socket from your tool chest, it may work as well. If things are set too far back, you may need an extension kit. In most, you still use the same trim, it extends the shaft (but not on all).
If acidic water messed up part of the thing, I'd seriously consider replacing the whole valve and likely the trim as well. Old trim rarely works on a new valve and vice-versa.
I know that I need the whole valve; the diverter is broken in half & the front half came out, so water spurts from all 3 holes...
I rather like my valve handles & the bezels are the kind of heavy chrome they don't make anymore, so I really don't want to double the price buying trim parts of lower quality.