wendyg,
Your questions 2 & 3.
If a replacement were to be required, it would be usual when a parade "fell in" for the NCO (or whoever) to call out, "Step forward any man who can blow a bugle". Thus, in this manner would potential bandsmen come forward for interview.
My father described to me how he was transferred from the Lancs. Fus. to the Royal Engineers when he was in Egypt in 1915. He was on parade when the CSM called out "Step forward any man who can do shorthand/typing." My father was just such a person. He took one pace forward, was tested, transferred to the Royal Engineers, posted to the EEF HQ in Cairo and eventually went with Allenby in the EEF's advance to Jerusalem.
He told me that he was reluctant at first to step forward, as some NCO's enticed applicants to come forward for what seemed a congenial job, only to discover that the task was peeling vast quantities of potatoes or similar. Regards. PhilipG.