Blocking the plate or a base without the ball is only obstruction if it "impedes the progress of a runner."
Many runners make the mistake of altering their path or slowing down too far away from the fielder. It needs to be clear they beat the ball there and the fielder impeded them.
So, based on what you're saying SoCal Dad, if the defensive player doesn't yet have the ball and the runner makes contact with her and/or otherwise goes out of her way at the last second to try to get around her, then there is obstruction ... but if the defensive player receives the ball before the runner arrives, she can block the plate/base, right?
Correct on both counts.
If the runner slows down or stops before she gets there because there is no place to go, then she can be tagged out without recourse, right?
That's a judgement call by the umpire whether the fielder impeded the runner prior to getting the ball. My experience is they won't call it if the runner eases up or alters their path early because the change is either not noticed by the umpire or they feel it was premature because the fielder still had time to get out of the way.
If that's the case, in the instance of the suicide squeeze,
the catcher should position herself in front of the plate because she in effect has nothing to lose. If she's not blocking the plate, pretty much the runner is going to score anyhow on this bang-bang play, right?