Danze valves aren't poor quality. They are one of a few that still uses an all brass valve body (even Grohe has switched to an all plastic body) and Danze has a pretty good warranty program.

It's 15 years old and subject to heat and cold, water and minerals. Shit wears out.

I get your frustration, but you have already spent more time and effort on this than it would cost to have a plumber come and fix it.
Time is Money LC, and being inconvenienced for days along with the frustration just isn't worth it over principle and a few bucks.
YMMV though.
You got 12 years out of a cheap Valve that was installed originally, sometimes sense overcomes principle.
Personally? I would have cut that thing out, bought a couple fittings and sweated a new valve on there and called it a day.
That's what i will be doing tomorrow for a reefer water supply valve, it isn't really very hard to do. If i hadn't gone cheap and easy in the first place, i wouldn't be doing that tomorrow.

The Joy's of home ownership.

Hot AND cold are killers. And minerals here in the NW are harsh and make nasty buldup that orings and rotating sealed parts HATE.

Shower part arrived this morning. Was a remarkably simple re-installation.

The bulk of the aggravation was the fact the part has NO identification and there are so many similar non-compatible parts on the market by many companies. And no local hardware store carries it, and only 1 in the entire state several hours away.

Once I identified it, which took several hours online looking at images and luckily finding a close approximate and then locating a thread, with a company, which led to a call to that company, and a helpful rep, who ID'd the product as a different company... A call to that company landed the product ID along with a lifetime warranty. Product was shipped same day and arrived Monday morning while I was enjoying my coffee - free.

Installation was simple from there. Great to have my non-leaking and clean shower back again (was impossible to keep it clean with a steady leak).

Lessons learned:
* Start these projects on Monday/Tuesday. Starting mine on Thursday put getting the parts more slowly by a few days.
* Prepare for a longer project than necessary.
* Frustrations aside, the employees at both companies were incredibly friendly and knowledgeable. And Danze warranty service was better than anticipated and as good as can be - free product and immediate shipping with no hassle.
* Recorded all information (company, model, dates, phone numbers, web site, warranty info) on a note card in a plastic bag duct taped to the pipe inside the wall.

Very nice.