Thanks for putting a word in, all of you. I guess I really needed somebody to tell me I wasn't just making up excuses about the venue.
The original plan had been to avoid showing the chairs at all costs, but the reality was that I had about forty minutes (which, long story short, would be over an hour before the concert started), little space to spare, and the wall behind the piano was lamer than I could have imagined. It was an on-the-spot instinct to reverse directions, wedge my back against those (locked) doors (visible behind the pianist in the second shot) and shoot towards the empty chairs.
While like all of you, I was not thrilled by the idea of all those empty seats, there were literally no human beings to put in them and in any case, my priority was to try to make the pianist look as good as I could in the time allotted and suck up any flaws of the setting that I couldn't hide. The posed shot of Ángel is one of the only two I shot of him from the audience perspective that kind of worked, and only after a lot of cloning out power outlets, wires and bits of a really ugly and intrusive lamp. Empty seats or not, unfortunately the chair background shots were among the best I got of him playing.
Far as Alvina the soprano goes, I'm straight up embarrassed I didn't get a single outright good shot of her, especially compared to the first shoot I did for them. The one posted just reaches "kind of salvageable in B&W considering how awful their old pictures were."
Peter, to answer your technical question, I managed to use three lenses over this rushed shoot: 70-200mm f/2.8, 50mm 1.4G and 55mm f/2.8 AIS Micro.
The first two shots were made with the zoom racked out to 70mm. Third shot was with the 55mm (still on after doing some hand shots I didn't include here) and last was the 50mm.