I'm also travelling all the way on the HC from INV to KGX. I couldn't reserve away from coach M. I assume all EC services to/from Inverness are extremely busy.
On the basis of "if you can't beat 'em [on this interminable thread!], join 'em, here goes.
I treated myself to a free 1st-Class London King's Cross-Inverness & return journey last week [using-up accumulated "Rewards" points!].
The EC booking engine put me in coach "M" on both the northbound and southbound Highland Chieftains. Remembering the advice of several on here, I thought I'd altered my reservation to Coach "L" until I printed it out, and found I was still in Coach "M" I assumed I'd make a mistake with my attempt to alter it, until I read some of the more recent comments on here.
Come the day last Tuesday, I found 6 or 7 reservations together in coach "M" on the 12noon off the Cross, so found a seat at a table for four in coach "L" which was unreserved, and enjoyed most of coach "L" to myself all the way to Inverness. The conductor didn't bat en eyelid at my moving seat, and the train crew were most attentive.
During the journey, I enjoyed Cumberland sausage & mash, several rounds of chicken and stuffing sandwiches, even more rounds of egg & cress sandwiches ["they'll only go to waste, Sir", the waiter said], five cups of tea, two cups of coffee, three pieces of cake, two bags of crisps [and a partridge in a pear tree - No - sorry!], three glasses of red wine, one gin and tonic, and two cans of "Old Speckled Hen" before putting me and my liver to recover in an early bed in Inverness at 2030.
I spent the following day in Inverness [actually going to Portree courtesy of "Scottish Citylink" [if coach enthusiasts want a resume of the journey, I'll gladly oblige!].
The following day found me watching Inverness station come to life at 0740, waiting to be admitted to the 0755 to the Cross. Expecting to be able to move again into coach "L" and relative solitariness, I was amazed to find almost every first-class seat in all three coaches reserved [mainly by people travelling just to Edinburgh and partaking of the Highland Chieftain breakfast, so had to sit with everyone else in my allotted seat in coach "M"
Nevertheless, the breakfast was superb [eaten in the warmth of a "Mallard"-refurbished HST while watching the snow-ploughs and crawling traffic on the adjacent A9 - it doesn't get any better than this - and I was actually offered
three pieces of toast in all!]; followed by potato and stilton flan after leaving Edinburgh, along with as much tea/coffee, alcohol and bottles of Harrogate water as I could manage, to arrive at the Cross spot-on at 1551, and not wanting anything else to eat or drink that day.
Very well worth the 0.10p it cost me! [it actually cost nothing - that was the price marked on the tickets.] Well done, East Coast.