There is no supported way to programmatically select all rows in a data grid. Currently built-in widgets do not expose a public API.
What about adding an enum with an image of a x or a tick. Add add microflow buttons that selects all by selecting all in the microflow using a retrieve and updating the enum to selected (tick) and a microflow button that deselects all, again select all in the microflow and set the enum to unselected (X). it work visually and you can use the enum in processing