Oh no! I’m so sorry, I thought you were the same anon who sent the later ask saying you’d figured it out! I didn’t mean to give you the silent treatment. XD
You know what’s awful? I can’t remember who Tessa is. To be honest, the episode’s case plot bounced around so much that I failed to follow it very well (dear god, Sherlock, just leave the storytelling to John, you’re incomprehensible), so I didn’t have much luck tracking people by name. For example, I can only remember the actual villain of the episode as Murder-Guy. So I’m not sure I can be much help on this one.
I can tell you, though, that training doesn’t necessarily stop people from leaving stuff lying in indiscreet places. Believe me: training is not a magic bullet that imparts unfailingly-followed perfect habits.
I dunno how John/Sherlock/Mary got hold of Sholto, either. I just let that one get handwaved, because if Sherlock wanted to get a letter to somebody, I’m pretty sure it would happen. Then again, I think I had the impression that John was still in touch with Sholto via email? He certainly knew a lot about a guy who’d gone into hiding somewhere in the Welsh wilderness (or wherever he planted himself).
Now that I’m talking about it, a little bit more is coming back. I didn’t think it was necessary for anybody to get hold of Sholto’s invitation, was it? I thought the premise was that Murder-Guy had stalked Sholto hard enough to guess that he would emerge to attend the wedding of John Watson, of all people. So what Murder-Guy really needed was to figure out where the wedding was being held. And if you know who you’re looking for, it’s not that hard to track down where they’re holding their wedding. People don’t generally consider that to be a closely-guarded secret.
I don’t know why Sholto was last to sit, either. I don’t recall it registering as a particularly significant detail. Did that matter? (Of course I can’t remember when the hell it was that he was supposed to have gotten stabbed, either, so maybe it did.)
You may have the impression that I hit Stag Night and basically forgot anything else was happening in the episode…and you may be right. I’m sorry to have failed you, anon. Hopefully someone else can come to your rescue in the replies/reblogs!