If neither I, nor my partner, actually saw our opponents' shot land out, may we still call the ball out based simply on our estimation of its trajectory?
I would say no. A ball is assumed to be in unless a player definitely sees it out. So if nobody is looking and sees where the ball actually lands, then I do not believe you can call that ball out. Any doubt has to go in favor of the ball being good. And in this case, there is some amount (albeit possibly small) of doubt created if nobody sees the ball land.
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