Those 150 or so plant nutrients said to be present in good
quality castings must obviously be supplied by the worms'
human caretakers, either in simple elemental or complex
molecular form.

Would feeding already rich mushroom compost -- already
brimming with organic goodies -- to my earthworm bin produce
an even more rich "super" vermicompost? How about adding
kelp meal, which should ensure a complete dose of all necessary
secondary and micro nutrients?