At the greatest confluence of all, Cairo exists - but is for some reason overshadowed by St. Louis and Memphis.
St. Louis is itself located at the confluence of the Missouri and Mississippi Rivers, for one thing. Although the Missouri isn't quite as important as the Ohio, it still has value for reaching the Great Plains. In any event, between St. Louis and Memphis, both of which were founded before Cairo, the city did not have an overwhelming advantage. Probably natural environment (being on a low, flood-prone plain) has played a role as well.
-Houston: If Galveston isn't the main port of *Texas, then Houston will snatch that role up
Galveston will never be the main port of Texas in the long run. It's just far too exposed to hurricanes to survive for long, especially if you haven't invested in multiple defenses. If the Dutch settled it and really wanted to make a go of it, I suppose it might work, but it would be much easier to just move to the mainland.
I'm curious why you think Houston is superior to, say, Beaumont as a port. Both, so far as I can tell, required substantial artificial improvements to be useful (in fact, this seems to be the case with Texas ports generally, either because the natural harbor is not sufficient to host modern vessels or because of other natural hazards like hurricanes)
-Salt Lake City: Totally dependent on the Mormons settling in Utah.
Isn't Salt Lake City in one of the most habitable areas of Utah? Yes, perhaps without the Mormons there wouldn't be significant settlement of the state, but given non-Mormons extensively settled other areas of the Mountain West...