I missed that write up on Trail Scotland - Often wondered what the Loch Ericht shore path would be like, despite being thoroughly warned off it! I'd have a punt of it based on that....
That said, Thomas, I think you'd be better on the Bealach Dubh on your gravel bike. That write up was done in July, it will now be very boggy whereas Beacha dubh is all on a well drained gravel path (albeit with a fair few rocks and drainage channels).
Re the rest of your route (the short one) - Looks like you are doing the Capel mounth from Glen Doll to Glen muik and staying in the bothy and then doing the single track past Lochnagarr.... err don't, really, trust me on this (at least not on a gravel bike!) From Glen muik I'd follow the gravel road right out to Ballater then the back road to Balmoral and a good track to Braemar. The feshie geldie will be a swamp so a gravel bike will be no hindrance - prepare for a long wet walk however.
Next up, I'd actually miss out Ben Alder and head from Ericht lodge up by loch pattack across to Lochan earba and then up strath Ossian and over the road to the isles to loch Rannoch - this is all top gravel bike stuff. The bit from Rannoch station to Kingshouse is a no-no - it will be a complete bog fest, actually dodgy given how wet things are just now. Instead from the south side of loch Rannoch hop over the Lairig gallabhaich to Glen lyon and then up past loch lyon and over to the West Highland Way and back to tyndrum (This is the Highland Trail route in reverse from Loch Rannoch) The West Highland Way south of tyndrum is truly horrible - lots of hike a bike (so maybe a gravel bike is your best bet!) if you do do this and exit at Inversnaid, mind and do all of the forest roads in Queen Elizabeth forest park, instead of your road route to Aberfoyle.
Your route out of Aberfoyle is another death march - instead follow Sustrans NCN 7 north over to loch venechar and to Callander - this is a great route. From Dunblane, to Bridge of Allan follow the NCN (can't remember the number - 74 maybe?) before picking up NCN 76. NCN76 is more scenic on the north side of the Forth, ie don't cross the Kincardine bridge, keep on to the Forth bridge (and visit Fife!)
Ordnance survey maps on bing.com/maps should enable to you to suss out this little lot - good luck!
