The bolded parts are the big problem with what you're arguing. The biggest misconception you have is that Dominicans are supposedly "of European descent" when the opposite is true. The percent of the population with Black ancestry is almost identical to Haiti's, something like >95%. More of them are mulatto, and esp because of the long dictatorship under Trujillo, many were enouraged to deny their Blackness and pretend or pose as European or white (when not even Trujillo was), but they really aren't by American standards.
American standards were occasionally pretty flexible. I think that the perception would be that the Dominicans were latins rather than blacks, and I suspect that's how it would be sold. I don't think that the Dominicans consider themselves anywhere near the same ethnic heritage as the Haitians.
That's true now, but more importantly for your timeline, it definitely was considered true back in the days of the one drop rule and needing to be 15/16 white or more to not be considered Black.
Again, American construction of race was a little bit more fluid than that. It's tempting to see it in those (ahem) black and white terms. But reality was always a bit more complex. Indeed, different American states had different laws defining race. One could be legally white in one southern American state, and be transformed into a legal black simply by crossing a border, and vice versa.
My view is that the United States, as a result of contacts with Spain in Florida, and with Mexico through Texas and later annexations had some basis for making a racial distinction between perceptions of latins and blacks. Statehood for a Caribbean latin state would have probably forced some minor mental reshuffling, but that's about it.
No way would a Democratic Party in 1876, who just concluded a corrupt deal to end Reconstruction, and Republicans who just sold out their Black voters in the South, allow a colony in the Caribbean with so many Blacks to become a state. The same will hold true with largely Black or mixed ancestry Panama, or largely mestizo and Black Nicaragua should the canal be built there.
This was certainly the case in our timeline. But never underestimate human flexibility, or the ability to say up is down when a vested interest is discovered to do so.
A Dominican Republic seeking entry to the United States would almost certainly want to enter as a state like Texas rather than a territory. Had they made it, they would have been a political, cultural wedge to redefine race concepts, at least in the caribbean.
I can see much of the rest of your timeline coming true, but not for any reasons having to do with Dominican statehood, instead economics or a military presence.
It's not actually my timeline, I'm just visiting here.
The end result might be more of a naked colonialism, instead of the pretence of it being for the good of the stability of the region.
Or perhaps a small polyglot of Central and Caribbean latin speaking American states. A backwards third world, backwards as the American South. And let's not have any doubts - the American south was a third world nation by any reasonable measurement, with far more in common with El Salvador than New York.
I'm interested in the notion that a significant Hispanic rump in the US congress might have influenced the politics of the day in measurable ways. Obviously three, or even five or six spanish majority American States in Congress wouldn't have been decisive. We're looking at 6 to 12 senators at best, and a likely smaller proportion of Reps.
So any influence or power that they might have would come from attaching themselves to historically existing power blocs or movements within Congress, and seeking concessions or policies of local application in return for their support. Which means that there may have been some significant deviations in policy decisions in this timeline. Unfortunately, I'm not such a dedicated student of Congressional history circa 1869 to 1920 to really say for sure.
It does seem to me that incorporating the Dominican Republic in 1869 would have potentially opened up wedges in Congress. And that those wedges might well have lead to other American states being incorporated, rather than merely ruled. But this is uncertain.
Possibly, the Dominican experience would create a backlash. Creating new states during the slavery era was often a politically complex and controversial deal, because of the potential of those new states to upset the slavery/free balance. Remember Bleeding Kansas?
Racial issues were volatile enough that after the 1880's, there would be resistance to incorporating Caribbean territory - which is why Cuba was conquered but not kept.
But if the Dominicans crept under the wire between 1869 and 1875, would this increase resistance? If increased, the Spanish American war might not even have happened. Alternately, the wedge and local Dominican agitation in Congress might have seen Puerto Rico and Cuba admitted as states, and possibly Panama and one or more central. It seems to me that we'd have to be able to parse Congressional politics far more finely to decide which way thngs would fall.
Following up on this, how would Roosevelts New Deal have been applied, had the United States incorporated several hispanic Caribbean and Central American states?
And for me the biggest butterfly may well be WWI. How could any American President argue for self determination when they deny it to half the Caribbean?
You'd be surprised.
In terms of WWI butterflies, would American conflict with Britain instead of Germany had a happy outcome? Unhappy for Canada, potentially. But could or would the United States be prepared to incorporate an unwilling Canada? Apart from that, if Britain rules the sea, would the United States have had a significant impact in WWI? It's hard to fight in the trenches when you can't cross the sea. Would the US have been able to challenge British naval dominance, particularly a dominance supported by the French and tacitly by the Dutch?
I don't see this as a wank, but rather a slightly interesting examination of whether incorporating a Latin Caribbean state as part of the United States representative government might have had subtle consequences.