Holy BOZO! how'd that happen?
Another thing about the actual removal process is to remember that, while the retainer is hardened steel, the housing is soft aluminum. Careful as you may be, there will be some shards of aluminum to clean up.
I recommend grinding the sharp edges off the retainer in the case of re-installation as well as cutting a notch in the end of it for easier removal next time.
With the retainer up top, you might try a wee tad of lubricant in the groove and see if it'll rotate around to a proper position.
OR just take it out when you remove the turbo to drill and tap the manifold for your PYRO gauge thermocouple. That's the best idea!Oo.