Mr. Moffat strikes again. Great googly moogly, but the man has no sense of character! If I were Mr. Gatiss, I'd insist that he only be involved in the writing at the development stage and that I had full veto power over his suggestions. Perhaps that wouldn't help, though. Mr. Gatiss after all consents to portray the character that Mr. Moffat destroyed in this episode.
I must grant that "His Last Vow" is, unlike "A Scandal in Belgravia," at least a good story. Yes, my sister has a point: Mary's backstory is a little histrionic and entirely unnecessary. After all, if Magnussen wanted to threaten John, there's no need to go so far as Mary. All we needed was for Magnussen to know somehow that John shot Jeff Hope in "A Study in Pink." John was carrying an illegal weapon and he used it to kill a man when he was not a clear and present danger. (I'm not sure what words British law has for it, but I'm sure they mean the same thing.) Yes, we (and John) know that he was, but I defy a defense attorney to make "He was about to persuade Sherlock to take a pill that might be poisonous!" fly in any court on the planet. The court would find that Sherlock was a big (and legally sane) boy and if he wanted to swallow that pill, it was his own lookout.
I, however, am willing to let that pass. I rather like the idea of our favorite adrenaline junkie choosing a wife along the same lines as he chose a best friend. It has a nice symmetry about it and (amazingly for Mr. Moffat) is extremely consistent with his character. What sticks in my craw is Mycroft Holmes's dialog and actions throughout.
First, Mycroft would take Magnussen so very seriously that Sherlock would probably never have even become aware of him. If a man can suborn high government officials, he's dangerous, and Mycroft got plenty upset over Irene Adler's blackmailing a powerless royal in "A Scandal in Belgravia." In fact, I have trouble believing in a Magnussen in the Sherlock universe at all: Magnussens take years to gather the kind of power he has, and Mycroft would've seen him coming and neutralized him long before Magnussen reached Lady Elizabeth Smallwood or built Appledore. It only worked in Conan Doyle's universe because Mycroft was distinctly lazy and only dealt with matters that people brought to him and because Milverton confined himself to blackmailing noble ladies over their romantic peccadilloes and stayed out of high politics.
I considered Sherlock's accusation that Mycroft was under Magnussen's thumb and rejected it. Magnussen could not have started at the top: when he started gathering information on important people, he would not have been inside the loop enough to know that Mycroft was important. So Mycroft would've seen him coming long before Magnussen got to him. Also, as my sister points out, a blackmailer cannot expose someone when it is either practically impossible or the person don't care. What is Magnussen going to do? Publish it in his paper? Mycroft can invade his paper's computer systemhis evidence isn't on disk, but his paper and its website would beand wipe out the articles faster than Magnussen can write them. Even in the unlikely event he managed to get ink to paper, Mycroft could have a strategic fire set. And who can Magnussen tell without publishing? The police? Mycroft's parents? Mycroft is in control in a way Magnussen cannot compete with even now, let alone when he would've first been noticed by Mycroft.
Last, Mycroft's holier-than-thou attitude over murder is positively pathetic, given that Mr. Moffat is a legally sane adult of the twenty-first century and Mycroft is supposed to be an extremely high-level player in the political realm. To a veteran practitioner of realpolitik like Mycroft, murder is merely a tool. Mycroft probably ordered his first murder on his first day as the British government, if not much sooner, and now even he doesn't remember who it was. Sherlock wouldn't have to murder Magnussen now because Mycroft would've sent Mary (or one of her colleagues) to do so very quietly long ago. The only aspect of Sherlock's actions that I could see Mycroft being superior about is Sherlock's doing the murder in such a public manner. It would even have made a great line: "Sherlock, if you want to get away with murder, try not to do it before a hundred witnesses!" It would have been better for Mycroft, in the scene with the other government officials, to make some sort of comment about how he could hardly save Sherlock given the number of witnesses, rather than pretending he thinks murder is such a horrible thing. When Mycroft made his scathing remark about Sherlock's being a murderer, I was insulted because clearly Mr. Moffat thinks everyone in his audience is either a small child or an idiot.
I think the only way to save this episode (and thereby keep a rather nifty story), would have been to make Magnussen more like Milverton than he was. If Magnussen had avoided politics and pursued his schemes just for money (okay, maybe also for the power trip), Mycroft actually might not have been impressed with him as a danger. We could even have had Magnussen aware of Mycroft's power and intellect, to explain why he stayed out of politics. Of course, I'm having a little trouble conceiving what could possibly be so horrible in twenty-first-century Western culture that people would want to hide it unless it was a crime, and Sherlock certainly would not be sympathizing with criminals and out to rescue them from a blackmailer. I suppose the exposure of extramarital sexual affairs would still qualify, as would events that were legally crimes but somehow justified (like John's killing of Jeff Hope, although you could get onto very shaky moral ground with such cases). I think Mary's backstory would still have worked (Sherlock would be for her just because John loves her, let alone that she's changed her bad ways). But Magnussen as he is and Mycroft as he is, no.