On the "may" vs "could" issue, it is normal to use the same word "may/may not" and so not doing that feels unnatural and adds a little ambiguity. In a true conditional sentence, the combination of "if" and "may" is unusual and makes the sentence a bit unclear. Do you mean: "If you like, we can go to the theatre, but we can do something else if you prefer."

both are correct, but the second one sounds better. I would actually say: "we may not go to the theater, if you prefer not to, but then again we may."