Contextual Filters Help - User Pages
I usually add a contextual filter for "user: uid", specify a default of "userid from the URL" and then set a validation criteria for "user" and "Allow both numeric UIDs and string usernames".
That way you can use both the uid and the username.

You'll probably have your "last post block" show up on the profile edit page because it's at "user/%/edit".
I changed my user profile pages to "member/USERNAME" using the pathauto module.