[Dixie republican]

if power can be extended over new people and territories then it likewise can be rescinded to allow a Voluntary mutually agreed upon secession.

If that were true, why didn't the Founders say so? They did a pretty good job enumerating what they wanted. You don't think they even considered secession?

[Libertarianize the GOP]

For the Same reason that most couples when getting married don't plan for their divorce. The option is there but you want to focus on the positive outcomes as much as possible to encourage that outcome. By simply granting the Federal Government the powers necessary to admit new States you don't have to explicitly State how those same powers could be used in secession. Also most all cases of Secession would not be mutually agreed upon and at that point any Constitutional provision would likely be ignored.