Any reason why you wanted to use that datepicker? It is a bit old. Could you try any of the others in the appstore like this one for instance: https://appstore.home.mendix.com/link/app/111485/ITvisors/React-DateTimePicker
Or just the regular date picker?
To edit objects in a template grid you always need to show another page to be able to edit the contents. So just create an edit button (as default button so a double click will open the page) and edit the object in that page. In that page you can just use the regular calendar widget.