There may be more than a dozen boat parties in the Grand Canyon on a

1. Either one could work. Of course, "others" is plural, but think of it this way: I could say "I didn't see any other person" or "I didn't see any other people." Depending on what else I was saying, one might serve better than the other, but neither creates trouble on its own.
2. The plural here leads to confusion. Each party is positioned in more than one way? What are these different ways? It seems we're describing one thing--a group gets positioned so that it isn't visible to other groups.
3. In everyday speech, it functions more or less like "in order to." ("I arrived early so as to get good seats." "I spoke in general terms so as not to exclude anyone.") The GMAT *could* use this; however, it doesn't seem to be the preferred idiom, and we've seen it struck out on an official question. I wouldn't rule out an answer just because it used "so as to," but if I had the choice, I'd go with something else.
4. Sure, "such that" is used well to show a result in A and B. It's what follows in each that creates the trouble.
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