It seems to be taken as a given in most ATLs that if some part of the slaves states secede by 1856 they will certainly succeed and if they secede after 1864 they will certainly fail.
A) Do you agree with this, at least during the 19th century?
B) Presuming no change in the Slave states (other than maybe Delaware), does this turn around once the Texas oil is found and a significant amount of the US war machine is fueled by it?
C) Would the addition of a Slave Holding Cuba (and possibly other Slave territories in the Caribbean) to the US significantly tip the balance?
1) No, at least not with any certainty. It is true that the earlier we're talking about, the better the odds are for the South to secede. For example, up until about 1848 the disparity in population and heavy industry between the North and South wasn't all that great, certainly nowhere near 3x that number as in the 1860s. However, to succeed the South needs to have a reason to leave (because of, or in spite of, slavery) be unified and all-inclusive; leaving out the Upper South means they'd probably still lose (making the Nullification Crisis as we know it a no-go unless some changes are made). Post-1864, I'd think that whatever legitimacy the region would've had would be gone by then, and even less likely to succeed without overt outside help.
2) You assume Texas would either exist, be part of the US, or side with the USA instead of the South. None of those factors are at all a given (although it's hardly impossible for Texas to stay in the union).
3) Cuba for whom? If we're talking about the South holding it, it depends on how strong their home-grown industry is. If they don't have any, Cuba's just gonna wither on the vine in the face of the USN. The "Union & Liberty" TL has Cuba being part of a Confederacy that is both IMO more "worthy" of existence (the war starting over a succession crisis in the White House, not Lincoln rising to the office) and has a stronger navy that can take advantage of the island as a forward base. However, this is far from a certainty.