Add and display a small text note per computer

Yes, it is easily possible using the Friendly Names / Comments feature, available under Options in the main menu.

The comment is a single text field that can be associated with a MAC address, IP address, or any other field. Typically, a MAC address is used as the key because it is unique and remains consistent for each device.

You can also create additional custom fields such as Location, Asset Number, or any other information you would like to store. This can be configured through Associated Data in the Friendly Names / Comments dialog.

Yes, please see the Friendly names and comments page of the Network Scanner User Manual.

You can simply press F2 on any device in the results list (main view) and assign custom names.