When I'm viewing a standalone post, currently the only way you can navigate back to the user's profile is clicking their name in the post header. Seeing how the sidebar has their name listed in big text as part of their profile, I usually end up trying to click that, first, before realizing it isn't a link. It feels like that name text - or the smaller one beneath it, or the profile image - should have the capacity to act like a link.
I might try whipping up a userscript for this at some point, but it's such a small change I expect the team could add it pretty trivially
Yes, ideally every instance of an avatar, display name, or username should be a link back to that user's page.