Macbook Pro 2012 MD101 cannot bootup, cannot reinstall OSX
It sounds like you had a replacement SSD that WAS working at one time and has since failed. You then had some work done on the Mac or had some diagnostics done (?) but with no results.

Trying another SSD yielded similar results: the drive was not found to install macOS onto.

I am re-stating your problem to make sure I got it right.

What it SOUNDS like is that there is something going on with the storage subsystem. A cable, a connector or something on the system board (disk controller or the like).

You could examine all the connections carefully make sure there are no kinks or breaks in any cables associated with the disks. And if you still have the original working drive I would be tempted to try that, assuming it is still around and still bootable.

Failing all of that I would say that the issue is with the Mac itself and if a repair shop can't fix it what you have is an expensive door stop, unless you want to use the Mac with an external drive plugged into it (assuming that works...)