Thanks for the drawing, Mike, and for the photos, Brendan.
Two things occur to me in viewing these items. First, the north switch on the siding could be moved north as much as 15 or 16 feet. Neither of our locomotives is more than 20 feet long, so there would still be 10 feet-plus of tail track. Secondly, given that the main line curves a bit to the west as it nears the summit, the train could be left on the switch, offering a little more clearance at the south end of the siding.
I'm thinking it would be nice to have some sort of platform at Top of the Mountain to allow passengers to detrain and observe the runaround movement, and perhaps some riders would enjoy a stroll down the grade toward the stream crossing or a walk into the woods. A simple platform comprised of a couple of ties and some 2 x 8 or 2 x 10 planks would do the trick. I've heard that at portion of our new property abuts that of the Sheepscot Valley Conservation Association, and perhaps we could link up with their trail.
One more quick thought. The temporary north switch off the main track should be firmly spiked and locked to the diverging position except when needed to access the main track north of the siding.