Pre-selecting default option in select type

Fantastic plugin!
I’m using it to register all my shortcodes and have come across a problem. How would I go about setting up a default option in a ‘select’ type? Not just choosing the first option of the array stack, but explicitly setting the default.

Also I have a mailchimp shortcode registered where I wanted to display a series of checkboxes for user to select multiple from so that the shortcode can print fields="firstname, lastname, email" but I cannot get this to work.

Lastly, some options do not appear to be editable once I try to edit the shortcode in the editor. Is it intended to be a once only use, or can settings be overridden when editing (i.e. the modal window doesn’t appear to fill out the values previously selected by user).