Reader's Digest version: the trail ran cold and of course, now Oppenheim is no longer with us. Chances are those outtakes are still in his personal archive. Who knows, maybe one day when we're all singing along with Carl & Dennis...
The serendipitous part of the hunt was the discovery of his reel notations which, when translated, proved most illuminating in confirming what was - and wasn't - recorded at that session, and in debunking the then-recent claim that the cameras captured a set-to between Brian & Mike over the lyrics.
Is there a problem with trying to work with Oppenheim's estate? I mean, if you said that the reels are most likely in his archive, then somebody should be trying to work through that "lead". I would also donate a considerable sum in the effort to finding and properly preserving and transferring the reels.