swap login link for username when logged in

You can look after the menu issue by using the if menu plugin. It allows conditional logic based on logged in state so you can show/hide menu items.

As for displaying the username, that’s a bit more difficult. You’d probably have to edit your template and add some PHP after the call to the navigation menu. It’s not too difficult to achieve but you do have to get your hands dirty.

Alternatively, maybe something like the User Meta Shortcodes plugin would provide what you need (although not in the menu).