MacOS – Cannot delete icon on menu bar

This is likely a status item, not a menu extra. (Not being able to move it around or remove it by holding ⌘ and dragging is the way to find out.)

In Activity Monitor (in /Applications/Utilities), set the pop-up menu to "My Processes" and look for anything that sounds related to the scanner vendor. Also, check ~/Library/LaunchAgents for a file that mentions the vendor in its name. Lastly, check System Preferences → Accounts → (your account) → Login Items.

I'm assuming by "desktop toolbar", you mean the bar at the top of the screen (properly called the menu bar), please correct me if I've made the wrong assumption.

You can add the eject item by opening /System/Library/CoreServices/Menu Extras/ in Finder, and double-clicking the file named Eject.menu. It should appear in the menu bar right away.

To add the battery indicator, open the Energy Saver pane of System Preferences, and check the Show battery status in menu bar box, near the bottom of the window.