As regards setting A as your default try setting a 'Select In Field'
action on your sheet, triggered on Active
i.e. bring-up the sheet's properties, go to the Triggers tab, press Add
Action for OnActiveSheet, add an action of type Selection/Select in
Field, and specify the name of the field & value.
I don't know of a way to stop the user from selecting none of the
values. But then, why would you want to, given that this implicitly
means that all values are in view anyway?
If no selection causes the table to take too long to render, then use onditional calculation to suppress the calculation of the table if nothing is selected.
On the chart properties tab for the table, enter
=GetSelectedCound(YourField) > 0 as the condition.
You can also change the error message from "Calculation condition unfulfilled" to a message of you choice by clicking the Error Messages button and overriding the default message with your own.
Hope that helps