1. On a foul popup, the ball must be catchable or there is no interference. If it is catchable, then the interfering runner is out and the batter remains up with a foul ball added to the count. This is true whether or not the ball is caught despite the interference. (I’m assuming the interference was unintentional).
2. Ground ball in foul territory that has a chance of rolling fair hits the runner in foul territory
a) If the runner unintentionally touches the ball: Foul ball (i.e., no penalty)
b) If the runner intentionally touches the ball: Interfering runner is out, batter remains up with a foul ball added to the count. However, if the runner was intending to prevent a double play, both the runner and batter are out.
3. Runner interferes with fielder on ground ball in foul territory that has a chance of rolling fair:
I’m not certain, but I think you wait to see if the ball rolls fair. If it stays foul, there’s no interference. If it rolls fair then I think it you would treat it like interference with the fielding of a fair ball: interfering runner is out and batter is awarded first base. (Again, I’m assuming the interference was unintentional).
Also, remember that if the fielder is just watching the ball rolling near foul line, waiting to see if it rolls fair, the fielder is not protected because the fielder is not trying to field the ball. In that case, don’t call interference (or obstruction) unless the runner’s or fielder’s action is flagrant or intentional.