Nice one Eric - a good trip and a good read. I came across the same problem with the bridge at glen Muick. Adam Wallace mentions this on the website - if you look at the GPX track closely there is a way down to the bridge which missed out the chopped off road apparently!
Further east the route is much less gnarly but still plenty of hills. Glen Gairn single track is my favourite trail in Scotland
Looks like you had a good trip and good to read about it.
Yes there's a wee track on the corner above the missing corner which you can use to short cut to the bridge at Mill of Sterin but you don't miss much, it's just doubletrack up to the same point on the path up to Lochnagar. You get to the path by going straight on at the first corner (ie before the switchback) after the road is blocked.
Eastwards on the Deeside Trail you missed a nice bit of short singletrack by Cambus O'May and a lot of hills with great views over Deeside and Donside. Glen Gairn is interesting, especially how isolated it feels despite not being far as the crow flies from the A93.