With Section III. the law is specific only to territories controlled by the Confederate Congress "lying without the limits of the several States". States are not controlled or administered by Congress.
Section II is clear, and refers to Citizens of each State, for the right of transit and sojourn with their slaves, the right of property in such slaves shall not be impaired. This means in States where slavery is restricted or prohibited by law either in importation or use, citizens with slaves can slaves can transit or temporary stay are allowed. If it were a requirement in the Confederacy for all States to allow slavery without question, then this section would not be necessary.
The Confederate Constitutional Convention allowed for admission of non-slave states by the overwhelming of rejection South Carolinas demand of admending the Constitution with the following words...[To amend the third section of the fourth Article by adding to the first clause thereof, the words: " Nor shall any State in which African slavery does not, by law, exist, be admitted, without the consent of all the States, expressed through their respective Legislatures."] These debates are hard to find and I've found coverage in the Northern papers, with speeches. I do think I need to find more on this. It would explain why the fire-eater rhetoric is found less and less after Fort Sumter.
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David Upton