Yes to the leaders of the Southern states or at least most of them. Slavery was merely the pretext - economics then as now was what really drove things.
Yes it was a power struggle. Votes in the senate mattered a lot (for those economic arguments) so long as the Southern states were in. Once they were no longer in the US, there was no need for votes in the Senate and no concern with spreading slavery. The power struggle was over.
It was the era of Manifest Destiny and that held true for both the US and the CSA. Were there bound to be some expansionist sentiments in the CSA as there had been and still were in the USA? Of course. That’s imperialism/expansionism. That’s not some holy crusade to expand slavery. Cuba already had slaves.
And the reason why they gave up and left the U.S., leaving the power struggle behind, was because of slavery. Which brings us back to the initial error pointed out in the plaque.
It was the era of Manifest Destiny and that held true for both the US and the CSA. Were there bound to be some expansionist sentiments in the CSA as there had been and still were in the USA? Of course. Thats imperialism/expansionism. Thats not some holy crusade to expand slavery. Cuba already had slaves.
And the Confederate Constitution guaranteed those territories would be slave territories and the states made out of them would be slave states. Non-slave territories and non-slave states were prohibited. Which is why the Corwin amendment held no interest to them; it didn't protect slavery to the extent their new constitution did.