saki101

...impressed by Sherlock's ability to play that quartet alone,

I went back and forth on this matter because I wanted to use that piece of music, but there are several problems with that as you mention. Since Sherlock is amazing, he could play just the first violin part but it raises two separate questions of why would he? First, why would Sherlock practice playing parts that need to be played with others (three others?!) since he is very much a soloist? (If John were musical, I could easily imagine duets though. I read a very good fic where Sherlock plays a duet with his mother who plays the piano. On that level they communicate, but not on most others.) Possibly when he was young and learning, he performed with others. Possibly Mycroft played an instrument when he was young and perhaps their parents as well and they played quartets. Still that leaves the problem of John then not hearing the full piece. Sherlock might have the rest of the music in his head, but that wouldn't benefit John and the serenade is for him. So after your question made me go back and consider what I had done, I decided that Sherlock has adapted the piece for solo violin on his own and changed the sentence to reflect that. How well that could be achieved is another question, of course, and beyond my depth to know. I noted that there were adaptations for two instruments and for piano, but I didn't see any for one violin. If it's musically possible, of course Sherlock could do it. But even if it is musically possible, is it desirable (except in my head because I liked the music)? It is considered so perfect, would tampering with it be right? Sherlock would probably consider himself up to the task, but still. So, the problem will remain. I should have chosen a violin solo really.

The morbid connection is easier. Apparently, Schubert admired the piece so much that he asked it to be played for him on his deathbed.

Regarding the lieder, I went back and forth as well. Originally, I was thinking that he played two or three or at least started more than one and then was dissatisfied with the lyrics distracting him. Then I thought that even two would take too much time if they were having that effect on Sherlock, so I settled on one and didn't make the sentence agree. I might never have noticed that as I have re-read it and it just didn't click, so thank you again for telling me.

Yes, the next phase is Sherlock stretching out on the sofa and waiting for John's response.

If the story ended up in rondo form, that was serendipitous!

Thank you for reading with this level of care, it helps me clarify my thoughts going forward and to correct a few things along the way!