Is 'Come hither, Leonato' meant for Benedick's ears or as a signal to start the gulling? 
In this scene I don't think that it means either. Don Pedro has the control in this scene as can be shown by his addressing of Leonato and that after he has said that he says, 'What was it you told me of today? That your niece Beatrice was in love with Signior Benedick?', Don Pedro needs no prompting from Leonato and proceeds to start the gulling on his own. However without Leonato, Don Pedro would be unable to do this as he has to incorporate him as it is his niece but this doesn't act as a signal but more of an indication that Leonato is an accessory to Don Pedro's gulling.
I also don't believe that it was for Benedick's ears because it the actual line holds no significance to Benedick whatsoever, I think his eavesdropping would have been aroused by the mention of his name within the second line.

Does Benedick react to this, if so, how and how large is his reaction?
Benedick does react as '(Benedick is still eavesdropping)', if he wasn't concerned about what was said then he wouldn't continue eavesdropping therefore he must show an interest. However he does not intervene in the conversation which could reflect of his character as the 'bragging soldier' as he has heard one nice thing about himself, he doesn't want to disrupt it and wishes to know what other things have been said about him. On the other hand, as love is a sensitive topic for him, he may be waiting to hear more as he needs reassurance because he can't bear to make a fool of himself in case he got it wrong.

'Is't possible? Sits the wind in that corner?' - is this a moment for audience contact? 
I don't think this a point for audience contact but a moment where he must have some self reflection upon what he has just heard. The audience at this point must watch the actions of Benedick as this is his own 'little bubble' time. Audience contact would also diminish their overview as it would mean they are in the same position as Benedick and therefore being gulled also, which ruin the humour at him being gulled.