Kevin, this is very interesting! I had the same issue with my GEM28 (except for the gear switch), and I can completely confirm the beneficial effects of your proposals.
Unfortunately, I seem to have a further severe problem with my GEM28. While trying different DEC backlash adjustments, I noticed that in some circumstances the DEC gear switch on my GEM28 would not engage in a specific rotational segment of the DEC axle anymore. I opened the DEC housing and measured the distance between the DEC worm gear and the DEC gear while the gear switch was disengaged. The distance varied during a full rotation of the DEC axle by about 0.2 - 0.3 mm. This lead to the conclusion that the DEC shaft in my case seems to be slightly tilted, wherein the tilted DEC shaft kind of staggers while it rotates, thus varying the distance between the DEC gear and the worm gear. I confirmed that the distance variation is not caused by a geometric/dimensional aberration of the DEC gear itself by also directly measuring the distance between the outer circumference of the DEC shaft and a fixed reference point, say a specific location on the gear housing. Result: The same variation of about 0.2 - 0.3 mm. This makes it quite difficult to find the right DEC backlash adjustment, i.e. the sweet spot, between an engagement between the DEC gear and the DEC worm gear that is not too tight when reaching the smallest distance therebetween and an engagement between the DEC gear and the worm gear that does not have too much play when reaching the largest distance therebetween.
Not knowing if the tilt of the DEC shaft can be corrected by adjusting the shaft nut of the DEC shaft at all, do you or anybody else know if there is a trick to loosen the shaft nut? I suppose, a special tool is used for "normal" assembly (which I don't have). I tried to use a pair of scissors to engage the two openings arranged on the outer axial end face of the shaft nut with the tips of the scissors. Unfortunately, I was not able to rotate the shaft nut since it seems rather firmly tight.
man I have the exact same issue with my gem28, have you fixed it?