"So this is yet another case where the Church was conforming to public opinion?"

Slapping kids is not "public opinion" in Germany, Kurgan, it is "public practice" (among half the population) -- though outlawed in public schools in the 1980's. Slapping choir boys, however, was not the "Church" norm throughout Germany or elsewhere. Some choir masters did, others didn't. Making the practice of a few (who conform to public "practice" or your notion of "public opinion") intrinsic to the whole "Church" is a false principle. The number of church institutions alone (globally, that have children's cathedral choirs  -- think of the Vienna Boys choir, Westminster's) makes for a different consideration or comparison.