What evidence is there that Hamlet is in "the interrogative mood"?Maynard Mack said it is, but I do not know why.

Maynard Mack did indeed write that Hamlet is "pre-eminently in the interrogative mood". What Mack might actually have simply said is "Hamlet is a play obsessed with asking questions". Some arguments to support this (I think, fairly indisputable) reading of the play might begin with the fact that the play opens (is it the only Shakespeare play do to so - anyone?) with a direct question: "Who's there?". That initial question endlessly reverberates down the play:

•  The ghost (of course, the character in mind when the question is asked) comes in "questionable shape", and the play expresses real doubts about whether it is a spirit of health or goblin damned. Shakespeare never resolves whether it is in fact the spirit of Hamlet's father or not.
  
• In the same way, Claudius can "smile and smile and be villain" - in other words, can act in a way which does not fit with the way he appears.
  
• In the same way, the actors visit Hamlet - it is , of course, their job to appear other from that they are. 

Shakespeare encourages to question what we see - and what appears to be. Everyone is spying on everyone and trying to interpret (Hamlet watches Claudius at the play, Polonius spies from behind the arras, Claudius and Polonius watch Hamlet...). The whole play is full of "questionable shapes" - nothing is certain. 

And - of course - the play is full of actual questions. Not least, one that begins "to be..."