I don't know the answer - sorry. But to address your point "How does fuse 5 come into play during shifting that would cause it to blow? It doesn't seem to have any interconnection with the shifting."

It may well not be involved in shifting, but something else that is? maybe able to short out the F5 circuit. Have you had a good look at the actual shift lever for wiring damage caused by the movement of the lever itself?

Does the fuse blow immediately you attempt to move the lever or when it selects a gear 'D - N - P - R' etc. Does it do it in Sport (or before you get that far? This may help to understand if it is the shifting action or the gearbox response that causes the fuse to fail.

On a previous X5 I owned somebody had attempted to chop into the loom to hook up a camera (I think) this had the result of truck occasionally refusing to shift of shifting up when I selected down etc. I recall reading of another case on a Mercedes forum where somebody had dropped coins down there and they were causing strange issues. Fizzy or sugary drinks are another thing to check out for, best removed with electrical switch cleaner. Good luck.

What code scanner are you using? My recent experiences show huge differences between scanners in terms of what they actually see and how much use they are. My icarsoft tells me I have no errors when clearly the car has an error. My air suspension being a prime candidate for this. Icarsoft shows "Okay" INPA shows errors 20 & 22 (sensors failing)