We all have favourite books that we feel get mangled by their film adaptations. But, as David Churchill points out in his perceptive piece below, the art of adaptation by its very nature is an imperfect one.
The alone part was a problem for Hitchcock (he introduced Madeleine Carroll's character, Pamela, who reluctantly -- she's handcuffed to Robert Donat's Hannay -- goes on the run with Hannay) and it was a problem for Mickery if she was going to be faithful to the novel. There is only so much that can be done with a character running around the highlands of Scotland without another soul in sight for large swathes of the narrative. Mickery was very faithful to the novel, to a point, but even she had to insert a female companion for Hannay, otherwise it could be argued, the film (and any adaptation) would be, no pun intended, handcuffed. The insertion point of the female character (Victoria Sinclair, a suffragette, played by Lydia Leonard) to join Hannay on his journey was believably chosen. (Hannay crashes a political debate pretending to be one of the candidates and meets Sinclair when he tries to escape.). The decision, I think, was a wise one because it was the only way to solve the large problem at the centre of Buchan's novel where Hannay would be otherwise alone. Perhaps Mickery's later decisions regarding whom or what Sinclair was were not perfectly thought out, but at least it gave the TV movie an emotional core that was otherwise missing from Buchan's novel.
The League of Extraordinary Gentlemen was another story. It should never been made into a movie in the first place if it could not be at least somewhat faithful to the graphic novel, and in reality it couldn't be: Moore took characters from the more fantastical novels of the late-19th century -- The Adventures of Sherlock Holmes, Dracula, King Solomon's Mine, Dr. Jekyll and Mr Hyde, 20 Thousand Leagues Under the Sea -- to create his 'league' who would fight international baddies at the behest of the British government. The biggest problem the adapter (James Dale Robinson, who had few credits before this and none since) faced was that if he were faithful to Moore's graphic novel, the film would have had charges of racism leveled against it. Neither Moore nor illustrator McNeill is racist, but the conceit they used to create the piece was a problem for any screenwriter. Moore wrote the book and O'Neill illustrated it as if it was a real British colonial 'Boy's Own Adventure' written in 1900. Moore assumes the voice of a 1900-era British writer who clearly holds the popular English view of the era that those in the 'colonies' were obviously inferior in all ways to the British. The cowardly adaptation was such a whitewash (the plot was changed entirely, lead characters were relegated to support, other fictional characters from the era such as Dorian Gray and Tom Sawyer were added that made no sense to the ideas Moore was working with) that Moore insisted that all future adaptations of his work (including the very-faithful-to-the-graphic-novel Watchmen) have his name removed.
The art of adaptation is tricky. For every success (the Bourne movies, which wisely jettisoned everything but the basic premise; the recent TV series Flashforward, based on Robert j. Sawyer's good novel, kept only the central premise and a handful of character/plot developments; Ian Fleming’s Casino Royale) there are hundreds of disasters (David Lynch's Dune, Quantum of Solace, Green Zone). Sometimes changes are made out of necessity to make the project work as a film (The 39 Steps), sometimes the changes are made out of fear of offending (The League of Extraordinary Gentlemen). Adaptation is an imperfect art and nobody will ever be fully satisfied, but as Mickery proved, you can still remain faithful to the source and the problems can be solved without pretending they don't exist.