Lost a game that way last year when our runner was called out for same exact thing. Would have probably called her out. In the slo-mo you can see the SS pause when the runner flashed in front of her. It's a tough one for sure, but letter of the rule it's interference.
Runners do not have the unlimited right to just run to the next base. They can't interfere with the person fielding the ball. I'm not saying this is interference as the angle doesn't show it clearly but it could be. In you example about the runner on 1st going to 2nd they can run around the fielder fielding the ball but can't just run into them.Do the runners not have a right to advance directly to the next base? I would think they should be allowed to advance if they are running straight to the base and not making an attempt to interfere. If their is a runner on 1st what is the runner supposed to do? Stand there to let the ball get fielded and get tagged out. I would think she should be allowed to run hard to 3rd.