I had a similar logistical nightmare a few years ago when doing an off-road ride from Garmouth (start of the Speyside Way, about 30 miles east of Inverness) to Fort William. In the end, I decided to drive to Dalwhinnie (on the Glasgow to Inverness line), leave the car there and get the train to Elgin, ride to Garmouth (about 5 miles), to start the ride. A couple of days later, my ride passed through Dalwhinnie so I picked up my car and drove across to Spean Bridge (on the Glasgow to Fort William line), dropped the car off and got a taxi back to Dalwhinne to continue my ride.
When I eventually got to Fort William, my plan was then to get the train back to Spean Bridge to pick up my car and drive home (though I ended up getting a lift there instead)
Dalwhinnie is about the nearest point in the Highland between the two lines without having to drop too far south. The taxi cost about 40 quid but was still the cheapest & least time consuming option* (Dalwhinnie to Spean Bridge is about 35 miles).
I wouldn't necessarily recommend this because, as others have said, booking that many bikes on trains is the problem. I booked 8 weeks in advance and still they (Scotrail) managed to f88k up the booking. At one point, a guard asked me to get off the train because others bikes had been booked AFTER I had booked mine on…
*Thinking about this again, there is probably a PostBus option between Dalwhinne and Spean Bridge but I was leaving Dalwhinne on a Sunday so no PostBus that day…