in case of this sentence meaning "Do they understand, Mara?" Which morph represents the 3rd person plural? Is this a case of an omitted prefix due to the subject "they" being in 3rd person? But why was my first answer "Does he understand, Mara?" wrong then? Or does the 'a' as an augmentative suffix give a hint about a plural 3rd person?
"Does he understand, Mara?" could be a possible translation of this if Mara was being called as the name of the person that was being spoken to. We have tried to be consistent, in this course, in marking such usage off with a comma. In normal conversation, context should make it clear whether you are talking to or about Mara. Since there is no comma here, for this sentence we are talking about Mara and not to her.
Yes, the absence of a prefix (or "the null prefix" as we often call it) indicates a third person subject which can be singular or plural. Thus both, "Do they understand, Mara?" and "Does he understand, Mara?" would be possible translations if the comma were present.