[Rich Ives]

What is the baseball equivalent of this?

There are several different sets of baseball rules. All have it covered.

Must an umpire ask for help, such as plate ump asking base umpire about a swing?

The only one that has a must is the Official Rules (OBR) of pro ball which is also the basis for most youth leagues. The only place ther is a must is when a checked swing is called a ball - then if the catcher or manager asks theumpire must check with the BU. Other rules sets say "may'.


Or is there any occasion where one ump can overrule another?

No.

Or may a coach ever take an appeal directly to the second ump and get a ruling?

No

Does the crew chief ever have authority to set aside any decision made by another ump?

Only if two umpires make opposite calls on the same play. Then he decides which is more likelly correct. He cannot set aside or overrule any call other than it that particular case.