Should managers/leaders have extensive knowledge regarding body language?

Actually yes, because most of the communication is done through non-verbal means rather than verbal ones. So, a manager or a leader should try to understand the body language in order to communicate well, but should not just focus on body language and leave the obvious message that is coming from someone's mouth. 

Knowledge, yes

Comprehensive, No, not really, this is for HR analysts to work on and not for managers, managers need to be trained to control their body language so they wouldn't give the wrong signals, but not to read others.  

Of Course, As they can lead or send some motivation by body language. And they Can understand the reaction of the person by his body langyage 

yes,

body language is a very strong tools you can have to get information and answers words didn't reveal, from both employees and customers.

specially the new one with no trust relation yet

Yes, because body language contributes more than 90% of a communication message which is of course extremely higher than verbal delivery. So if body language fails to align with the message you are delivering, then there is a high chance for your message to be failed. 

sure, because it will help both way.

reading signs to get answers  and get information that words do not tell from the customers and the employee "team members " specially the new one. 

Absolutely, because body language comprises of 55% of communication in face to face communication whihc is much larger that words said 7% and tone of voice 38%. Therefore the manager should be able to read correctly what is not been said by words and question or take a cue to follow through.