I lost a metal part on my N62 engine in the same spot you lost the chain tensioner (upper oil pan). It took me forever to fish it out, and you are right, very difficult to see, etc. I believe I tried for a few futile hours before finally fishing it out.
In my case, just before I almost gave up, and had the same expen$ive thoughts, I went in for a final sweep, and this time, I went very slowly on top of the lower part of the chain, and right when I touched the crank, I felt and heard the "click". It was lodged between the crank and chain.
So, if you haven't tried that spot, I encourage you to do so, because right when I was close to give up, just like you, I was sweeping the whole upper oil pan, EXCEPT where the chain goes around the crank...and there it was.
It's probably the only place you haven't probed.

GL

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.