Overall, you're absolutely correct.

This problem boils down to the modifier (Architects and stonemasons). A modifier can be thought of as a simple aside, or as a set of adjectives that describe something.

The caveat is that the something being described must always be close/next to the modifier. For this problem, only C & E put the subject close, and only C makes any sense. Remember, these are adjectives, NOT the predicate, so the verb 'were' has no place there.

It sometimes helps to rearrange the sentence and change the words to get a handle on what's important. In this case:

E: 'Musicians and showmen, Metallica set the tone for serious metal bands in the 80's and beyond.'
vs.
C: 'Musicians and showmen, were Metallica..."

Note how much the comma makes C the incorrect choice. Since the modifier error is so glaring, this problem becomes simple, since we don't have to worry about the correctness of the rest of the sentence. There is only one CLEAR correct answer.

One last note, while the meaning is supposed to be maintained, occasionally it will be altered since the word order MUST change. Think of this not as the meaning changing, but as the true/intended meaning becoming clear rather than fuzzy.

Ben 'Riding the Lightning' Miller