OK, I went exploring…
Approaching from the Brunstane direction, I went through the new housing estate at Newcraighall. That makes for a handier shorter route for NCR 1 (nice), but, umm, how on earth did the Clowncil (they deserve that here) not get the developer to pay for a zebra or toucan crossing across Newcraighall Road? There's nothing apart from fairly poor sightlines and your own luck. Serious fail.
Along the path alongside the A1, you realise just how substandardly narrow it is compared to more recent construction, which is a genuine shame as the link at QMU is good and wide and smooth in comparison. An obvious missing link here: in theory it links QMU with Newcraighall station, but it is narrow and unwelcoming, and if they're serious about it (let alone serving Newcraighall itself) why didn't the Borders railway project add an underpass/tunnel and a lift from/to the station platform to the east side of the railway where people actually live? It really is such a "can't get there from here" for something you can practically see from there and adds at the very least 5 minutes' extra walk to get to/from the station unnecessarily. And there really should be a burgher bike dock at the station so you can get to QMU or (hah) the retail fortress just to the west (if they cared about such things).
OK, back to the route. I'd forgotten what a slog uphill it is from the underpass under the A1 to Whitehill Road. There's now a gate across the path where it meets the road, but with a gap for bikes next to it (I guess to try to prevent flytipping?).
Down to the waste plants. The gap between them shown on OSM is still there, but the side road stub almost immediately turns into gravel and mud, sort of rolled out for about 50 m, but then it's overgrown with plants beyond that. You'd probably need to hack back a bit even to get through on foot, so not a route for bikes, even pushed!
I followed the road on the east side of the waste plants further south, which quickly stops and becomes a track. At the northeast edge of the woods, it turned out that there is a very obvious (in the black coal dusty ground?) track leading into the forest heading west south of the boundary fence (which, annoyingly, has an inaccessible nice perimeter road going all the way round). This track looks well used, either for BMX or perhaps motocross, heading further into the woods. Anyway, it extends through to the west side of the woods (although sufficiently uneven that I had to push), and so you can reach the semi-derelict roads on the west side. (Maybe someone with OSM skillz could add this?)
It's a little hard to get your bearings or scale here as it seems that much of the ground here is covered in concrete, presumably part of the former use. You head south a bit, and then west, along a well used looking track to come out opposite the entrance to Shawfair station, although, where the track doesn't quite reach the road, you have to roll partly down into and up out of the (dry) ditch alongside the road.
Amazingly, from the station access road the route to Loanhead and Roslin is well signposted from here onwards, although the bridge over the railway to Newton village (over which you are directed) has the most awkwardly angular and tight hairpins on its ramps, making it almost impossible to cycle up or down (would it really have been so difficult to spend a tiny bit more and make the turns looser and more curved?).
And then onwards, via a rather horrible and fast looking probably hostile two lane roundabout onto Shawfair Avenue, which, infuriatingly has a decent wide cycleway alongside. So why end it with a horrible roundabout here? Guess you weren't serious about encouraging future residents east or north of here to cycle after all! Stupid!
And finally onto the cycleway to Straiton. Quite nice, through open countryside for the first part, with some nice wildflowers alongside. It is a steeper slog uphill than I had expected until you get to Gilmerton Road, however, but maybe that's a heavy 3-speed for you…