At the time of His Last Vow, I thought the show really couldn't dip much lower/more outrageous as when they turned Mary Morstan Watson into a CIA-trained rogue assassin.
But that was prior to Season 4. I had yet to find out that, indeed, it could get a lot worse.
Interesting that Moftiss chose to make Mary an assassin, because in my view, that's what they've done to the character of the woman whom ACD depicted as John Watson's guiding star and the love of his life (if that spot wasn't already taken up by Sherlock Holmes. Who knows if Moftiss is guilty of gay-baiting or not in their intentions for their screen pair, but I am speaking of Holmes and Watson as platonic best friends/halves of the whole/frères du guerre.)
Moftiss's Mary was essentially a smear job on Watson's beloved wife. No offence meant to Amanda Abbington, who was as much in the dark as we were until the Unmasking of Mary in that 3rd epi of S3. She was blindsided, same as we were, and admits that she would have played Mary significantly differently had she known the Secret. Which is probably why Moftiss didn't tell her. They wanted their actors and their audience on the back foot. 'Cause, you know, S. Moffat is a megalomaniac that way. Anyway, j'accuse him of being such. Uber-controlling, that one. I thought it was a shoddy way to treat his actors AND his viewership . . .but he'd just proceed to continue blithely on doing so in TAB and the whole of S4.
Essentially, having blown it with Sebastian Moran in TEH, the third episode of that season reintroduces elements of Moran with the 'empty house' motif--only *Mary* is standing in for Moran. Moran is the cold-blooded paramilitary assassin with the air rifle who makes it his mission to shoot Holmes. What a can of worms they opened up with this variation, though. Because in 'The Six Thatchers', they have to explain Mary's background as a G.I. Jane/ninja assassin. Her death at the end of that episode is gutting for John, but it's fitting, really--Mary lived as a soldier and she died a soldier's death. Mary had to die, because we know from Canon that this Mrs. Watson is doomed . . . but what a convoluted way round the houses to get there. To add that extra layer of verisimilitude, (unbeknownst to us at the time), Martin and Amanda were already estranged by the time they came to enact the Watsons' final, permanent separation. Talk about Art imitating life!
Makes me wonder if they were already having problems during the filming of 'The Sign of Three' when they had to enact a joyously married couple. Amanda looked so beautiful as a bride; it had to be a source of no small irony, as they spent what must have amounted to several days' worth of filming the wedding/Best Man Speech scenes, that this couple had never actually put a ring on it in 15+ years of togetherness.
Tough breaks for Amanda A., who I liked as Mary. I did not like what Moftiss did with Mary, though. They made her go through her paces like a stunt pony until they disposed of her in the story. Perhaps *she* could have been the long-lost Holmes sister (non-psychotic variant) . . .and I would have felt gobs better about the final two seasons generally.
I can tolerate TEH, am generally fond of TSo3, but could seriously do without HLV. I love TAB (one of my top four favorites, I think). In general I like T6T far better than TEH, and like TLD well enough (except for John kicking Sherlock), but I largely dislike TFP even worse than HLV (those two are in my bottom three, right below TBB).
So I agree with a good bit of what you say, just not all of it.