I agree that the runner is entitled unobstructed access to the plate...unless the catcher has the ball (or is "about to receive" it) as was the case here. Still, the runner should have tried to avoid the tag by sliding to the outside of the plate. That is smarter base running.
Sure doesn't look like she blocked access to the plate to me. Incorrect call and bad slide. Slide away from tag and she may have legitimantly been safe.
Yes. In that case, all 10 would bat and any 9 players that you choose would play defense. The players can change positions at any time. "Free substitutions" kind of, but it technically is not substitution as you would have no substitutes.