And it’s wonderful that Sherlock does recognize that Trevelyan needs him as a big brother, but he feels insecure about that role.

My own headcanon for this verse is that Sherlock in general panics whenever he has any sort of responsibility placed on him, and with Trevelyan, there's a lot of guilt from abdicating the role of big brother while he was drugged out. Which is why he goes to such extremes to keep him safe, while failing at any other aspect of being a big brother. At least in his own mind, where as Q thinks he's not too bad, really.

(Lysander’s my favorite, of course, because anyone who can keep Sherlock entertained has my undying love.)

I'm glad you enjoyed him. His adult self may make an appearance at some point. I like to think that he and Sherlock keep in loose touch. :-)