How can I empty PART of my trash bin without emptying all of it?

Sort by date by clicking on the header DATE. Click on the first one to delete. Move to the last one and hold the Shift key while clicking it. This will select that block of messages. Press Delete. You can hold the control key and click random message to select messages not together or to add to the block you selected doing the above. You can hold the control key while clicking a selected message do deselect it.