"...'till River kills the Doctor."
I realise that this has been picked over quite a bit, and that I'm late to the party. Forgive me; I've had stuff going on.
The Wedding of River Song was a fun episode, certainly. But, while I accept that it was never going to live up to its promise as far as fandom was concerned, I think there were a couple of promises that it should have lived up to.
Firstly, there's the promise that we were given in The Impossible Astronaut, by Steven Moffat, speaking through Canton Everett Delaware III (the older, blue-eyed model). Amy suggests that the Doctor we've just seen being killed might be a clone or a duplicate or something, and Canton - a character we've never seen before - replies: 'I believe I can save you some time. That most certainly is the Doctor. And he is most certainly dead.' Now clearly, given later events, he's either mistaken or lying, since the Doctor is a duplicate. But the way the character's presented, that statement is effectively a promise from the author that the solution isn't going to be that easy. In dramatic terms, the character's function at that point is a form of Lampshade Hanging or a Signal From Fred (warning: TV Tropes site - following links from there may eat your weekend).
For the conclusion to be at all satisfying, therefore, then either Canton need to be shown to be right, and there to be some other solution, or there needs to be a plausible reason why he's wrong. Neither of those happen; he's just straightforwardly wrong, and that for me is a broken lampshade/promise from Steven Moffat.
Secondly, there's the hanging question from last season: why does the Doctor getting put into the Pandorica cause the universe to vanish? There were hints in Doctor Who Confidential that that might be addressed this year but - unless, I've missed something - it hasn't been. And it was quite a big plot point in The Pandorica Opens, made more significant by the fact that the alliance against the Doctor basically thought that the opposite was true; that not putting the Doctor into the Pandorica would destroy the Universe. An explanation - more of an explanation than I think we got - would have been nice. Maybe we'll get it next year, but I'm not holding my breath.
I was prepared to be scathing about the Doctor's means of surviving his assassination at Lake Silencio without causing time to collapse. After all, he circumvents a fixed point in time by substituting a robot duplicate for himself. On the face of it, this suggests that the problem is not that (for example) he survives to continue to have effects throughout the timeline that are inconsistent with it maintaining its integrity, but that Time is a sentient, sadistic observer who stalks off in a huff unless zie sees someone looking like the Doctor get shot down at exactly that moment. Which is getting towards the silly end of fairy tale logic, for me.
Yes, I get that he tells Dorium that he's going to lie low. But Dorium himself points out the trouble with that; it's simply not credible to think that he's going to be able to avoid being noticed forever, now that he's unexpectedly alive when he shouldn't be.
But then I noticed that I was making the same mistake that Dorium was making. The same mistake that the Silence were making. The same mistake that the majority of people watching a series called Doctor Who might be expected to make.
I thought it was about the Doctor. The Silence think that the Doctor has to die at Lake Silencio to stop him being able to answer a question they believe will destroy the universe. Naturally, they think that the fixed thing in the equation is that the Doctor has to die.
But it's not. It's that River has to kill him.
The Doctor will continue to have an effect on the timeline, but time will cope with it. It has up to now, after all. But River Song? If River hadn't killed the Doctor, she would at the very least have continued with her archaeology career, exploring who-knows-what places in the universe, quite possibly with the ability to time travel to boot. River is awesome. River is also a loose cannon. She's a child of time with unspecified Time-Lord-like powers and years of conditioning and training to allow her to stalk and kill the Doctor. She leaves valentine messages for the Doctor dotted throughout space and time, and is quite happy to deface/create famous historical landmarks to do it. In an earlier incarnation, she nearly killed Hitler on her first trip. It seems to me entirely possible that if River Song is left free to roam the universe, she may cause irreparable damage to time (although with such flair and panache I'd probably still love her for it).
So, to stop time collapsing, River has to be imprisoned in the Stormcage facility. Obviously, she can (and does) leave from time to time, but possibly because she travels with the Doctor for a lot of that, or possibly because her career is curtailed, she doesn't to it enough to bring about the end of history (or whatever-it-is that 'happens' in the episode). The event at Lake Silencio is a fixed point in time because of how it affects River, not how it affects the Doctor.
I think the Doctor understands this. I don't think he's daft enough to think that letting the Tesselecta get shot in his place would fix the timelines otherwise. I'd like to think that River comes to understand this too - I can kid myself that I can see echoes of her understanding in her conversation with Amy at the end of the episode. I have no idea at all whether Steven Moffat and the production team understand it but, well ... they only made the episode, didn't they? It's up to us viewers to find meaning in it. ;-)
(Aside: I'm not happy with this from a River-as-awesome-person-with-agency point of view. But I think the episode's already got rather a lot of problems on that front anyway.)