Have finally sorted out my travel arrangements for this. Going up, I couldn't get a cycle reservation on any train for Friday so am travelling up on Thursday - the benefit is to have a free day for a ride on Friday, before the event.
Going back, it seems impossible to get a cycle reservation on the Monday - it looks like there will be engineering works between Perth and Edinburgh that week, so I've booked an early train from Inverness to Perth and then a 5 hour gap to ride the 44 miles from Perth to Edinburgh (which I remember from the Mille Audax) before getting a train back south.