Well, we made it there and back.
The good bits (mountains, snow, hotel, not actually getting killed on the A82, the night duty manager feeding free nice coffee to an early morning insomniac, pool, the breakfast, cafes, Scotrail staff, Alpine Bike handing over a 40 quid track pump and letting me walk out of the shop with it to inflate CA's repaired front flat without so much as blinking, the ferry tour in the sunshine, the ferry people sticking our bikes in their office while we were on the boat, the views, the bridges etc etc etc) were unutterably lovely.
The bad bits (no sleeper train for half the journey north, the crap train that replaced it, arriving so tired that it hurt, the A82, being completely incapable of actually making a plan to ride m'bike anywhere, 8 hours of being freezing and miserable in London, just London and being there with a bike which for possibly the first time in my life felt like an encumbrance that I wished I just didn't have to deal with, and feeling rotten at inflicting all of that little lot on my sister) were indescribably shit.
I think the good outweighed the bad, although I'm not sure by what margin - ask me when I've slept. For about a week.
The best bit? CrinklyAuntie. Who is a sodding hero for putting up with me.