WMB, my friend, you are making this way too complicated.
On the situation you presented, you don't call interference any more than you call OUT on a batted ball that goes directly to the catcher's equipment and is caught by another fielder - even though for any other batted ball rebounding off of any other fielder's equipment to be caught by another fielder would be a catch. We don't call this a catch and an out because by definition, it is a foul ball.
We do not have to sweat the definition of a play here because the rule gives the exact scenario... a fielder is attempting to field a batted ball while the batted ball is over foul territory and is interfered with by a runner.
The rule tells us what the fielder is doing. It tells us what the runner did. It defines the result. FOUL BALL.
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Tom
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