It is possible that John Buchan believed that the overthrow of the Stuart Dynasty in 1688 was not totally rightful and that the genealogical heirs of the deposed King James VII & II retain a considerable right to the throne and that he desired that they could somehow regain the throne.
I point out that in the opinions of many persons the length of time that a dynasty is deposed does not decrease their right to the throne, and i give examples of dynasties regaining power centuries after they were allegedly deposed.
That does not necessarily mean that John Buchan ever plotted with other persons to violently or illegally overthrow the then government in the UK. It is perfectly possible Buchan hoped that the Jacobite heir would someday regain the throne through a peaceful and legal process.
And I show how the Jacobite heirs could regain the throne without a violent revolution.
What difference does it make how long ago a dynasty was overthrown?
If overthrowing the dynasty was rightful, the dynasty has lost all right to the throne.
If overthrowing the dynasty was a treasonous crime, the dynasty still has as much right to the throne as it ever did, and it is a treasonous crime to accept the regime that overthrew it as being rightful, no matter how many years, decades, centuries, or millennia pass.
Therefore, it seems perfectly logical to me that someone might claim that the descendants of Confucius, who was allegedly descended from the rulers of the state of Song, who were allegedly descended from the Shang Dynasty of China, are the rightful heirs of China, even though the Shang Dynasty was overthrown by the Zhou Dynasty about 1122 BC or 1046 BC.
The Zagwe Dynasty ruled as Kings of Kings of (part of) Ethiopia for about 370 or 133 years from c. 900 to 1270 or from 1137 to 1270. The last Zagwe king Za-llimaknun was killed in battle and replaced by Yekuno Amlak.
Yekuno Amlak claimed to be descended in the male line from the Kings of Kings of Axum or Aksum. The last monarch of that dynasty, Dil Na'od, was supposedly deposed by Queen Gudit about 960 and the founder of the Zagwe dynasty supposedly later deposed a successor of Gudit. Yekuno Amlak claimed to be descended from Dil Na'od and to be restoring the rightful dynasty to the throne after about 310 years. He also claimed that the Axum or Aksum Dynasty had ruled Ethiopia ever since it was founded by Menelik I, son of King Solomon of Israel (reigned c. 970-931 BC?) and Queen Makeda of Sheba.
So the dynasty of Yekuno Amlak is usually called the Solomonic Dynasty or House of Solomon. During the Zemene Mesafint "Era of the Princes" 1769-1855 the Solomonic Kings of Kings lost all power and were deposed at will by powerful nobles. Yohannes III of the Solomonic Dynasty was deposed for the third time in 1851, leaving the throne vacant until the usurper Kassa took the throne as Tewodros II in 1855, beginning the succession of modern Ethiopian rulers from various families, most claiming to be descended from the Solomonic Dynasty in the male or female line.
But King of Kings Tekle Giyorgis II (died 1873) who reigned from 1868-1871 claimed to be a member of the Zagwe Dynasty which had been deposed 598 years before.
So Ethiopia is an example of a country where two monarchs claimed to be restoring the rightful dynasty to power after centuries.
And certainly the Stuart Dynasty isn't the only deposed dynasty, or the longest deposed one, whose heirs can be traced to the present time. The Regnal Chronolgies site has an appendix, Pretenders, listing the genealogical heirs - as well as the creator could trace them - of various deposed dynasties of various countries.
If you have ever thought nobody could possibly be the genealogical heir of a Roman Emperor? If so, you are incorrect. The eastern section of the Roman Empire, the so called "Byzantine" Empire, lasted until 1453, and the Holy Roman Empire lasted until 1806.
So I have started a thread: https://historum.com/threads/heirs-of-holy-roman-emperors.123117/2 and another thread: https://historum.com/threads/heirs-of-the-byzantine-empire.121359/3 discussing the genealogical heirs of various dynasties that ruled various avatars of the Roman Empire and claimed to be political heirs of all the previous emperors going back to Augustus the first emperor.
So there are many people alive today who can claim to be heirs of the Roman Empire.
A PEACEFUL AND LEGAL METHOD FOR THE STUART HEIRS TO REGAIN THE THRONE
Furthermore, as early as the first half of the 18th century, there was a faction of Jacobites trying to get a majority in the House of Commons so that they could vote to replace the Hanoverian dynasty with the Stuart Dynasty.
So why do you assume that all Jacobites want to violently overthrow the present dynasty instead of using legal political methods to restore the House of Stuart?
Or possibly Jacobites might seek to pass a law that the heir to the throne should marry the Jacobite heir as soon as such a marriage is possible in order to unite the two rival claims to the throne.
If there was such a law in the 1810s, for example, Princess Charlotte, only child of the prince of Wales (later George IV), could have married someone in line to the Jacobite succession, except that there was no bachelor of the right age who could be expected to inherit the Jacobite claim. In real history Princess Charlotte married Leopold of Saxe-Coburg-Gotha, later King Leopold I of Belgium, and died in 1817 after giving birth to a still born child.
Charlotte's first cousin Victoria (1819-1901) became the heir to the throne in 1830. If there had been a law requiring her to marry the Jacobite heir, she could have married Francis V, Duke of Modena (1819-1875), who was the jacobite heir (1840-1875).
If there was such a law at the present time, it would make Prince Charles (b. 1948) seek to marry Duchess Sophie in Bavaria (born 1967) who is third in line to the Jacobite succession, being the oldest daughter of her father who is the only brother of her childless uncle. Instead Prince Charles married Diana Spencer (1961-1997) and Camilla Parker Bowles (b. 1947) and Duchess Sophie married Prince Alois (b. 1968), heir to the throne of Lichtenstein.
A historical example of that process happened in Sweden.
King Gustaf IV Adolf was forced to abdicate in 1809 and his childless uncle made king by the plotters. That childless uncle eventually adopted the French field marshal Jean Bernadotte as his heir and the Bernadotte Dynasty now reigns in Sweden. But a marriage between the eventual heirs of both dynasties makes the present king the rightful genealogical heir of both the deposed King Gustaf IV Adolf and the Bernadotte Dynasty.
So all his descendants have to do is marry the heirs of about half a dozen other deposed Swedish dynasties and they will become the one hundred percent totally rightful genealogical heirs of every historic king of Sweden.
See this question: Who is Swedens rightful king considering the coup d'etat of 1809 deposed the then rightful king?4
And see the links in my answer.
And that is a historic example of how the heirs of a deposed dynasty can regain the throne legally and peacefully.