That's a detailed and interesting procedure, especially the part where the gearbox is assembled to the motor in a vertical orientation, clearly to avoid any actual or perceived influence to the alignment of the splines ... despite the presence of locating dowels intended for that very purpose.
The word "rumble" is interesting as I don't recall too many affected owners using that term. And does that mean or include "wheel-of-fortune", my favorite term, or clicking noise, etc? I suppose "wheel of fortune" doesn't look very professional in engineering documentation. "Wheel of misfortune" might be more accurate however.
If the noise was due to a lack of stiffness or undesired resonance of the motor end bracket why are not all examples affected? To me, the recordings we've heard don't sound like a resonance, more like a bearing failure because the clicks vary in frequency with the vehicle speed. Resonances by definition are fixed in tone, like a tuning fork.
The different size fasteners used after 13 May 2019 may not actually indicate a problem was found and resolved but possibly that a change was applied for lack of any better ideas. Once that change was made they are now stuck with accommodating that difference ongoing. Also note those fasteners are replaced with the motor and gearbox in situ, i.e. not positioned vertically.
There is no path in the flowchart to replacing both motor and gearbox as we know from experience seems to be the most reliable fix for the WoM noise. Nor is there any mention of checking the splines for damage.
My impression is that they still don't really know what the root cause is and are checking everything they can think of. What are the chances that it's nothing more than break-in metal particles in the gearbox damaging the bearings?