No advantage in leaving the box in neutral . I would certainly consider leaving the handbrake off. That's what I do with my old Bmw E46 cabriolet .

I'll be getting her out once a month to allow her to run for a good few mins.

Isn't that the worst thing you can do? Certainly, take it it for a a good long run every now and then, or just leave it alone?

I'd run it at least every couple of weeks for a good few miles. I don't hibernate my e class cabriolet but I have treated the roof to a good proofing and done the seals.