I agree. If we're comparing America to all of Europe, then Europe is still more decentralized, but if we're comparing America to individual European nations, then America is more decentralized.

As to the matter of US states leaving the union, it's very unclear. You're right that Texas is the only state that explicitly included that in their state constitution. At the time of the original union, though, it was generally accepted that states could legally separate and that was made explicit during the ratifying conventions. I don't think states would be forced to remain in the union today, if they attempted to secede.