Hm.
With the greatest respect, OP, I think you could do with a bit of assertion.
I wonder if deep down you are just a little bit pissed off that you do all of this work for free? How are you feeling about your boundaries?
It sounds to me like you are overextending yourself, exhausting yourself, for not proper compensation.
Be careful not to to fall into martyrdom.
You are human, you deserve recompense, you certainly don't need to take someone's 'fury' over what sounds (complicated, but as far as I can tell, fairly minor?) like a pretty inconsequential error.
I find that working pro bono pretty much guarantees we'll be treated like crap, interestingly, and any time we offer to work for cut-rates, etc, the jobs turn into nightmarish time-eaters. I'm no longer sure it's in the best interests of anyone to have that kind of working relationship.
Just throwing those thoughts out there ...
That said, the ability to handle a complaint well can sometimes actually do more for one's brand than you'd think. That doesn't necessarily mean self flagellation, just honesty and active listening.