The composition being played by violins is called a symphony, which it is not. A symphony is played by a full orchestra, usually in four movements.
(~23 min.) The Wikipedia page about the curse exists, but the text is different as of 2023.
When Barnaby and Jamie are questioning Ivo Baxter, who owns a music shop, Jamie correctly pronounces "Totenfeier" as "toe-ten-fire" (after Kam has told him), but Baxter "corrects" him to "tottenfeer". Someone for whom "classical music is an obsession" would almost certainly know the correct pronunciation.
Any professional musician ordered to leave a rehearsal, with the implication that they were not to return, would take their instrument with them when they went. They certainly wouldn't leave it behind in the rehearsal room, out of its case, and lying on a chair where it could be knocked over and damaged, as De Harthog does his viola.
At the beginning, a very high-level musical piece is being played. All piano concert pieces of this level would be required to be memorized, and a piece of this scope would require the pianist to look down at the keyboard and his hands, yet he is watching the music closely. This would be impossible to do, and certainly not acceptable for a performance of this caliber.