Can I buy tickets at Mann Center instead of Ticket Master?

You can absolutely buy tickets at the box office if they are available and you do not have to pay any extra fees associated with Ticketmaster. Check The Mann's website for hours.

Yes

You can at the box office. However depending on availability and the show I say

I would call but you can if available.

Yes, there is a box office at the entrance

Depending on the performance. Best to call the Box Office to be certain.