Going West to East, as suggested.
Glendrian - Branault (Ardnamurchan) boggy and 50% bikeable, views are the reward. Ferns and ticks a slight issue at this time of the year.
Branault to Kentra Bay 75% bikeable. Ferns really bad here as you leave Ockle.
If you don't take the above route you've got a lot of tarmac to leave Ardnamurchan.
The guided version of your ride linked to above seems to intersect with the 'traditional' C2C at Fort Augustus and the lovely Corrieyairrack.
The way I would do that is to go from Kentra Bay to Acharacle, hence to Salen to pick up the singletrack road north to the fireroad up the east shore of Loch Sheil. I've done that, but without an OS map in front of me I can't think of how to reduce the tarmac. You'd also have tarmac from Glenfinnian to Corpach before you hit Neptunes Staircase and the Great Glen Way to Fort Augustus.
The route variation taking in Luibelt/Loch Treig head Corrour is viable as mentioned above but I can't type about everything
Use Landranger maps and practice map reading before you go if you don't have experience. On a bike you can quickly cover the whole sheet of a 1:25000 so 1:50000 is better. I don't use a GPS but plenty of folk I know do, but have a map and the knowledge to use as backup.
MBA bothies on or close to your route at Glen Dhu Lighe, Invermallie, Blackburn of Corrieyairack, Ryvoan. Other bothies may be available (I'm a great fan of bothies).
>regarding 24hours - hasn't and can't been done for a reason. I would be an interested spectator if someone wanted to try setting the fasted time for the classic Ratagan-Montrose route - couldn't compete though, too old!<
P.S. Why a guided company are taking people East to West is beyond me.