I have a slightly more optimistic view of what is possible on laden tourers, I used to do offroad touring on cyclocross tubs. However some of the boggy stuff will be very hard.
I rode Kingshouse to rannoch years ago – the section marked as singletrack was a boggy mess of a quad track -impassable virtually on an MTB then. It may have been improved but I wouldn’t like to bet on it.
Last year we rode kinlochleven / Loch trig / loch ossian / then north up strathossian towards the A 86 and alongside Lochan na earba to laggan. That is all good estate road bar the single track stretch along the Abhain rath between loch treig and Loch eilde mor which would be a drag your bike thru a bog You could certainly get from Dalwhinnie to corrour via this route without too much hassle
I wouldn’t attempt the ben alder route – the bealach is very steep and rough on the east side –
I think Dalwhinnie to Corrour via Lochan na earba would be your best bet and probably the only feasable way of crossing between the rail lines offroad