Not to be a pedant...
The history of the Macnab is an interesting one. It was devised by John Buchan the famous writer in his book 'John Macnab"... where it consists of sending a letter of intent to the estate owner(s) that you will poach a salmon and two stags in a certain day... provoking a hunt of the hunter if you will. It is a classic for good reason, and I would recommend it highly to anyone.
This has evolved into what we have in Scotland today, which is an Atlantic salmon (on the fly), a brace of red grouse and a red stag. While there is discussion amongst those of us who practice these things on whether it is within one day (sun up to sun down) or a 24 hour period, I would say that a trout is never on the cards - they are too easy to catch! Poaching is no longer a part of the thing... perhaps sadly!
I am delighted that you mention the royal version, which is rarely heard of; but as you say is the same as the above, I've heard that you must sleep with the lodge cook, not the laird's wife though...
In my experience these things are the most fun when they are organic and not seen as a 'challenge' to be set upon with intent. If you catch a salmon before breakfast, what fun it is to take to the hill for a stag with a shotgun carried as well, and if the stag is shot... look for some grouse.
In recent years there seems to be an increased idea (predominately from our cousins across the pond) that it's a great challenge to be beaten. My own feeling, as one who has guided both types of event, is that this sullies the individual parts in the pursuit of a sum that doesn't matter. Sadly I have seen in some places this leading to stags otherwise far too young to be culled as part of a sporting management plan being shot. All this, I feel, would run surely against the characters in that most excellent novel.
Sam