Forget that Montana is special argument. The Constitution means the same thing throughout the country. As the supreme law of the land, it trumps anything in any statute, any enabling act, any treaty, any contract. There could be something in the Montana enabling act that guarantees its citizens greater rights to bear arms than are guaranteed by the second amendment, sure. But Congress could not do anything in 1889 to alter or evade or define whatever the second amendment itself means.
There can be no secession. The Civil War settled that. "One nation, indivisible, with liberty and justice for all"
I have heard about some weird stuff in some statehood admissions agreements. Like Texas supposedly has the right to subdivide into five states or something if it wanted to. I have no idea if this stuff is enforceable.
The actual issue in Heller is very interesting and debatable. It will be a split decision. I think the individual rights people might win. What is politically weird is you have the left-wingers who have advocated broad interpretations of every constitutional right for the past 50 years---and if you are consistent, that would mean broad individual rights under the second amendment. There are even some prominent liberal-Democrat constitutional scholars who concede that, yes, their own prior positions would indicate that the gun owners should win this one.