As I have a DD that catches, Ask 20 umpires and get 20 answers. For ASA rules the fielder at any base CAN NOT block more than 75% of the base. The catcher can not be established in front (blocking the plate) of the plate before the ball is thrown or you get the obstruction call. Once the throw is in the air the catcher may go to the ball if contact is made it is call incidental contact ( no call ). Once the catcher has the ball than everything is fare game. With that being said good luck trying to find two umpires to make the same call.
Incorrect, incorrect and incorrect. The only thing partially correct is once the catch has the ball....
For ALL rule sets I am familiar with, nothing in the rules say anything about a portion of the base must be left open. The rule also does not say anything about a defender may not block a base. Defenders may play anywhere they like, but, they may not hinder the runner without posession of the ball. Defenders can stand in front of the bases all they want, they just cant impede the progress of the runner if they dont have the ball. About to receive a throw is only an NCAA rule. If a throw draws a defender into the path of the runner and hinders them, it is obstruction. The defender is not in posession of the ball. Once a defender has the ball, yes then they can block the base all they want but not everything is fair game. They cannot commit an unsporting act such has an excessively hard tag etc.