It's easy to jump into a debate over stage rules and personal boundaries, but as I see it that misses the point on why you (OP) reacted as you did. (Dear reader, where I write "you" below, try to put yourself in that space.)
You write early on that you have a hard time standing up for yourself, meaning at least that you have a normal aversion to conflict. Faced with a social transgression, those of us who share this aversion tend to resent those who are insensitive to it, because it puts us in the position of having to initiate conflict to deal with it. So there's that.
What I relate to in the OP story is that the singer's thoughtless action implied personal disrespect. I have a relatively high sensitivity to this myself, while recognizing that most people are less sensitive. Never ascribe to malice where incompetence will do.
We focus on the social violation of someone handling the instrument when he should know the rules, but the rules aren't stated, and people tend to see rules as pertaining more to others than oneself. He believes he knows his skills in handling the instrument and his motivations in doing so are good, so no problem.
What got you steaming wasn't the violation specifically — in the end he did no damage, while advancing the work you were there to do — it was the perceived disrespect. This is an insult to identity, which goes very deep when we let it. Your emotional reaction makes sense in that context, but you were talking past that in ripping him a new one, which didn't make sense to him or the witnesses.
So yeah, an apology and explanation is warranted, clarifying your boundaries with reference to how you felt about his action. He then has the opportunity to affirm his respect without accepting excess blame for a minor rules violation, and you have a path to repairing the rift.
