The clause doesn't say "unless they leave", and you can't make it say that.
Your 'argument' is nonsense.
Walt
The point is not whether it says "unless they leave", the point is that it does NOT bind the states into the Union, as you originally suggested.
You'll have to dream up another fiction to bind the States to the Union involuntarily.
The guarantee is restricted to only States "in this Union". It is not an "unrestricted" guarantee at all, as you said. Those three words are a qualification of the Guarantee. You can't hallucinate your way out of that. Those three words acknowledge that a State can be in or out of the Union.
You can't make it say any thing about binding the States to the Union. There is nothing binding in that text. It pledges a service to the States that choose to take advantage of it.