The all 5 car idea would never have happened Inverness can barely fit a 4 car hst on platform 3 and 4
That's overstating it a bit isn't it?
The 4 cars can fit on platforms 3 and 4 with plenty to spare, as can be seen in this video at 10:45 or 11:40:
As far as I can see, there'd be no problem with fitting all the coaches of a 5 car set on the platform; the question of course would be the implications of the power car going beyond the signal.
I sem to remember some discussion previously on the thread that the signals at Inverness allow this on some platforms but maybe that's only 1 & 2 (I certainly remember watching the sleeper depart from those platforms, back when it had a load of motorail vans on the front, with the loco(s) well beyond the signals)