I don't know where to begin. You had no control over your artist? You're not his friend or playmate, you're his boss! Marcos has put on enough shows without a hitch that I'm curious exactly what YOU did to cause a problem.
You must never be late. If you are arriving at someone's home, it is permissible to be a few minutes late, but not to an event of your own. Personally, I would have simply hired a room at a hotel in Coral Gables near the gallery, had my team meet me there to dress me and finish my hair and makeup. It is disrespectful to your guests and clients to show up hours late to your own function.
When I saw your "plan B"...the thing you call a "happening" I wanted to reach through the TV and smack you. What type of low class person hires a painter to perform a flash painting at another painter's vernissage? As an artist and owner of 3 galleries, I couldn't imagine a greater insult or harsher way to disrespect an artist during a viewing. You didn't understand how insulting that was, you didn't understand that your guests knew just how wrong it was to do such a thing and you didn't understand that the man who was loudly complaining was in the right. Perhaps he should have been quieter about it, but what you did was so insulting to an artist I understand why he was so upset.
So, he didn't have all 37 (or did you say 32) pieces completed. At a vernissage it is often the case that a limited number of works (usually the best of the set) are present for a private selection of critics and customers. As I said, I've heard of Marco Marin before. As far as I've heard, he's had only minor issues before (although he's never shown in my galleries). Certainly nothing on the level of producing only half the requested number of commissioned pieces. I'm left to wonder if this is a case of you trying to place blame on someone else after your own mismanagement brought on a poor show.
Suck it up girlfriend. I think your nouveau riche is showing.