it's called a "Dremel challenge". We had the same with the previous AFS TCs. A few minutes work with a Dremel is all it takes.
I would like to still have the old mount, just in case I want to sell it.
I called around this morning, they are not allowed to sell me repair parts. Luckily, I was able to source a loose Z-mount from Mainland China, where residents usually own what they buy, thanks to places like Shenzhen. The mount looks to be a direct fit.
It is held in via 4 screws, each with blue threadlocker. I wonder how the FTZ/Viltrox et cetera adaptors will see this thing after I convert it. If it works well with older telephotos, I would like to make sure Nikon sells fewer Z-mount ones for all the trouble I had to endure.
The TC1.25x for the 800mm/E has a small bracket as a hindrance. That thing is easily removed, the camera does not see the TC1.25x. I think it will be the same case for modern Z-mount ones. As far as I know, I am the first to attempt modification on modern Z-mount TCs. If nothing works, I will just make a threaded mount.