A855 - 5 Speed

wow, I can't believe I just read this sad, pathetic, ginormous thread.
I know it's too late at this point, and I'm not a super-experienced manual transmission mechanic, but I'd like to offer a suggestion.
I think the video with the open-end wrench shifting the 3-4 shift was telling. I think I would have pulled the trans, secured it to a bench & by wrench-shifting, "fully" shifted it into 4th, then carefully pulled the side cover to see if the gear is FULLY engaged, then ACCURATELY measured the fork "handle" pins distance, and then check the distance between the holes in the interlocking levers to see if they agree. Then I would manually move the 3-4 slider back and forth to make absolutely sure the slider was fully seated in 4th gear, and remeasure the distance between the fork handle pins. The distance should be very consistent, provided the "rocker" struts are undamaged and working properly. My gut suspicion is that there is some imprecision in the machining of the side cover and/or detent levers if the struts are ok.
I would also replace the cover & wrench shift it to the "3-1/2" gear position, carefully remove the side cover and repeat the above procedures, and maybe something will become apparent.