In response to the OPs question about what can be done if it happens again:
When i went to see Streetcar at the Broadhurst, i was in a rear orchestra seat which had a curved seat in front of it that was pressing into my knees in an extremely painful way. I arrived about 15 minutes early, and after sitting there for 5 minutes, i realized that i was already in so much pain, i would never be able to get through the whole show that way. I was prepared to walk away and lose the ticket money, since the show was sold out and i didn't see any other option like aisle seats, but figured i might as well ask the house manager if anything could be done to assist me before giving up and walking out completely. It turned out that the box seats were not counted in the "sold out" description for that show, because they didn't sell them to people in advance, but used them for exactly the kind of on-the-spot situations that sometimes arise. They seated me in a box seat, and although i couldn't see the staircase where the famous "Stella!" scene took place, i could see a lot of the rest of it.
Maybe if a show is nominally sold out, a box seat might still be available, or as others have said, sometimes they have stand-alone chairs in the back for people with either size or accessibility issues that they can offer. So the general strategy would be to ask a house manager for help (ask an usher to direct you to a house manager). Some theatres also have standing room spots that might not be sold even if the seats are. Sometimes they have options that aren't obvious to the theatregoers unless they ask for help.
Also, sometimes sitting forward in the seat gives your arms side space in front of your neighbor's arms so that both of you can lower your arms. Perhaps you could alternate crossing your arms in front of you with scootching forward a bit when you want to shift your arm positions, and then scootch back again if you need more back support again.
The odor issue is harder, because even if someone bathes daily and uses deodorant, some people still won't like how they smell. Our local theatres all have scent-free policies, which means that you can't use body spray or cologne to try to mask your human odor (whatever amount of it gets through the soap and deodorant odours). I would think that in the unlucky case where someone is sensitive to a nearby scent, whether of the person or of their cologne or anything else, the above suggestions re: asking for help with re-seating should apply reasonably to those situations as well. It is tricky because some people will complain when you do wear a scent, and others when you don't, so there isn't a one-scent-pleases-all solution that one can deploy, assuming they are already doing the daily bathing-and-deodorant thing.
