Situation 1: If the ball, defense, and runner all get there at the same time, no interference. If interference, run counts unless the interference happens before run scores and 2 outs. Timing play.
Situation 2: if F4 is in a position to field ball in the base line, then runner has an obligation to alter path to avoid collision or hitting the ball about to be fielded. If a runner hits a player who has a play at the ball, runner is out. So the fielder stopping in the base line is perfectly legal, and if the runner hits the fielder, the runner is out. If the umpire thought, she'd have a double play, the umpire can call runner and BR out. May not seem fair, but that's the way I understand the rule.
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