For those following this thread.....good news, the "October version", due mid October, will have a "Clear all checkboxes" option on the Refresh Grid action as well as the Clear Screen for New Entry action. By default these options will be off, but you'll be able to clear a grid's checkboxes (all pages, not just the current page) using actions.
Mary - for the export grid, that screen is a built-in screen not made using Method actions. Plus the checkboxes aren't in a grid, they are just a list. So if the list was known, you could just Enter Value Into Field On Screen to False for each checkbox.
Paul