the call is "obstruction" not "interference", correct?
Yes, it is obstruction.
the letter of the rules state the defender cannot block access to the plate unless in possession of the ball.
Not correct for NCAA softball. While true for most amateur codes, NCAA does allow a fielder without the ball to block the base, so long as the fielder is about to receive the throw.
the commentator (Michelle Smith?) remarks that the "ball was on its way", but that isnt a consideration for obstruction.
"On its way" is not the requirement of the rule. The fielder must be "about to receive" which means that the ball is about to be caught, not just merely in-flight and on the way.
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.
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.The fielder doesn't get to determine which part of the plate the runner should try for. Leaving a tiny sliver of it unblocked, if the runner executes some certain kind of slide to some particular portion of the plate doesn't excuse a potential obstruction call.
The runner is entitled to an unobstructed path to any part of the base she wants to try for.