Shiplap panels

I'm planning to build a panel door (a gate, 34.5" x 62") with shiplapped boards inside a grooved frame. While I understand the basic layout, I have a few questions on the details:
As regards wood movement, should the boards be tight up against each other, so the whole panel moves together? Or should each board be held back slightly from its neighbor, so that it can move individually?
An obviously related question: my frame will have two z-braces, on either side of the rectangular panel of shiplap boards. Do I want to secure each board where it crosses one of the z-braces? Presumably if I do I need to hold each board back slightly from its neighboards.
As to aesthetics, is it conventional to treat the exposed corners of each shiplapped board, such as with a chamfer? Would leaving these corners square look odd?
Lastly, I need to cut a mail slot through one of the gates. I'm thinking of building a small frame grooved on the outside which would float within the panel. Is this viable?
Thanks, Wayne
P.S. I've always heard that wood shingles should be secured with only two nails, for reasons of wood movement. But it seems like the width of the shingle between the two nails is still trapped and could split from expansion/contraction. Is it just that one nail is structurally inadequate, and so two nails is a compromise? Or is it that it is OK to trap a small width of wood (e.g. up to 6"), and that therefore when nailing a very wide shingle, the nails should be set inward more than usual to keep them less than 6" apart?
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