Can't find anywhere what tag/shortcode can be used to set usernames appear in content like it is in the navigation bar.
Like Hi [username] in a page or post. Is it even possible? Something that is/would(?) be replaced by "Guest" for, well, guests
You cannot do that in the default software. You'll need an add-on that adds custom BBCode. I know there is one somewhere, but don't have a link right now.