Yes, I think that's right, Lola Red (sorry, I crossposted and hadn't read yours).  Sherlock is in danger while he's in hospital, because Mary has threatened him.   But he's also worked out that she'd rather not have him dead, so there is hope to resolve the situation - if Mary knows that John knows her secret, then Sherlock is no longer a risk to her in that way.

After that hurdle is out of the way, there's another situation where both Sherlock and John are now a risk to her (they could turn her in).   It could be that Sherlock still thinks Mary is a risk to them until they remove the risk to her - by accepting her as a client. 

I think it's also possible that Sherlock still genuinely cares for the "three", and wants to keep them safe and together.  He aligned himself with Mary at the wedding: "I know I speak for Mary as well when I say we will never let you down, and we have a lifetime ahead to prove that" ("lifetime" is rather ominous, isn't it?).  And then "... my first and last vow. Mary and John: whatever it takes, whatever happens, from now on I swear I will always be there, always, for all three of you".  (Thank you, Arianne de Vere).  If John was unable to accept Mary, then I suppose she'd have to disappear and John would lose both her and the baby.  And could they just wait around for her to attempt to kill Magnussen again and do nothing to stop it?  (I know Sherlock kills Magnussen later, but at that point, I don't think they know it's the only way to deal with him).  That all has to be resolved quickly - as long as Mary is a client she won't kill Magnussen.  I do get the impression that Sherlock thinks John and Mary can't sort it out themselves (and John wouldn't think of taking her a client without Sherlock there), and I don't know if it is something that can wait at that point.  Whereas once Mary is a client, it buys them some time.  And for John to get to that point, I think he needs to hear the story about Mary "saving" Sherlock. 

 

Last edited by Liberty (May 30, 2015 1:58 pm)

Especially as it's the title of the episode!  I'm not meaning to get into that debate - it could really go either way, and "his last vow" could be meant in an ironic sense.  I just think either scenario (Mary being a risk, or Sherlock protecting the three) needs quick action.