The select option on the staging records is a field on the staging. This is different from the selection or grid rows. For this scenario, you can even use the new Bulk editing in grids to mark staging records to be selected.
I read your blog, that's a great feature which is reintroduced and the placement of the bulk update button was indeed tricky to find out as I don't use those overflow menu in the grid to add columns or show footer.
But what will make this feature more useful is "Bulk Select" before Bulk update as it's a huge miss for bulk updating records. This is missing in the data entity also. Any tips if this can be customized?