You'll be able to see the bike in the not too distant future WS, and if I haven't taken the top plate apart, you'll see close up and I think agree that the slotted nut is on right. I can tell with the thickness of the part of the nut that is the "top" is flat up against the top clamp, and from looking down from above, I can tell there is no taper. The side to side movement feels right and is bouncy (laterally speaking) and as much as I hand tightened the cap nut, it hasn't moved.
Your tricky idea for the mirrors getting through the gate came to mind. I find myself doing that as-is if I don't enter the gateway dead center . . . by the way, what are you doing putting bikes in the living room? :)
Today brought a new thing to look at. A few oil drips on the ground, which I haven't had in two months. I'm going to clean the general area tomorrow and go for a ride. When I get back, I should be able to sort out where it's coming from. I noticed it on the rear left side of the oil pan. There was a little bulging bit of clean oil near the corner. Since the oil was clean with no grit or dirt, I'm hoping it just blew down from the oil dipstick cap. FYI, I park on the center stand. I had checked the oil and may not have gotten the oil cap tight. I just tightened, carefully. Actually, loosened, then tightened to make sure I didn't overdo it. I am changing the oil in 500 miles (about 5 weeks).
The second thing to look at is the oil switch which was new when the seller did engine work back in 2005. The pan gasket shouldn't leak. I checked all the pan bolts and they are tight. Looking around the bottom, there is some moist dirt in the normal places.
If I don't get the drip tomorrow, I'll know it was the oil dipstick cap. Otherwise, I assume from what the seller told me to look at, is the pushrod tubes.
Riding my airhead is like eating chinese food . . . an hour later, I need to ride again.
1978 BMW R100S "bitza" (R80/7 frame)