James , I've seen that reference to Olga having written a letter to the author John Buchan too. See my post, reply 172, on "Olga's Letters" thread. I think there's a fair chance the Imperial Family read "Freemantle". I'm skeptical of Olga having written something of a fan letter to John Buchan. First, I don't think she wrote to other than family and friends very much, if at all. If she did write such a letter it would likely have been iduring the IF's earliest months in Tobolsk when correspondence was relatively freer than it became after the Bolshevik takeover in Nov. ,1917. Private mail to England from Siberian Russia while both countries were at war (which they were during most of the Tobolsk captivity) might have been somewhat problematic.
Thus, again, I agree the theme and genre of Buchan's "Freemantle", do seem up the IF's alley, so to speak, but I'm not so sure about Olga's letter. It could as well have been something along the lines of the story eventually getting back to Buchan that the IF read and enjoyed his novel(s).
The War To End All Wars btw, was very good. As is The Thirty-nine Steps.