The "Recently Imported" Playlist is a dynamically created, updated, and deleted Playlist. It isn't even a Smartlist but is a Playlist built and modified by MC code.
If you remove all items from the "Recently Imported" Playlist, MC will remove it from the Navigation Pane. That is controlled by code, and you can't change that. If you want it to stay in the Navigation Pane, you will need to leave at least one file on the Playlist. Also note however that if you don't import anything for a period of time (no, I don't know how long that is, but a week or so?) all the items will drop off the Playlist and again, it will be removed it from the Navigation Pane.
If you decide to leave items on the "Recently Imported" Playlist so that it will persist in the Navigation Pane, you can make changes to the layout (columns) and they will persist, as long as you make the changes on the MC Server, and not on the MC Client. Those changes will then persist through MC Client and MC Server restarts. However, if the "Recently Imported" Playlist is emptied it will always revert to the default layout when it is recreated by MC.
If you want what you describe, create your own recently imported Smartlist (or Smartlists perhaps, if you want one per Media Type) and use that/those instead, ignoring the MC created one. See
https://wiki.jriver.com/index.php/Recently_Imported for ideas on creating such a list.