In my opinion, 10.7.1A does not cover the OP's situation. "B1 takes a certain spot on the court..." implies intent, an intentional act that is neither accidental, as in "falls to the floor," as in 10.6.1, nor incidental during a scramble, as in the OP.
I'm much more comfortable saying to a coach, "He didn't have legal guarding position," than "There's an old case...."
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