So the FW day coaches still need to be dropped on to the north end of the ABD/FTW portions.
There's a scissors crossover around the middle of the platforms at waverley. I was wondering if a strategy would be to split the Inverness section off immediately and draw it forward with the 92. Then perhaps it is possible to back the FTW coaches in over that crossing, while the 92 is detached and before the INV portion sets off.
Not enough room inside the scissors crossover for the extra coaches and locomotive to be attached. Process appears to be:
Aberdeen locomotive arrives in platform 20
Inverness locomotives arrive in platform 20
Fort William coaches plus locomotive arrive in platform 1
1S25 arrives in platform 2 / 19
Portions separated
92 remains attached and draws Inverness portion forward to clear crossover
92 detached
Aberdeen locomotive attaches to front of its coaches via West end of station
Inverness locomotives attach to the front of their coaches via crossover
Aberdeen locomotive draws coaches away from Fort William portion
Doors released for Fort William passengers in Aberdeen seats
Inverness portion departs
Fort William seats and locomotive shunt to East end and back through platform 2
Aberdeen portion departs
Fort William train formed and passengers can board
Locomotive draws Fort William train east to platform 2
Runs round via platform 1 and crossover
Fort William portion departs.
The problem is that none of the complicated Fort William stuff can happen until the Inverness leaves.