I'd expect a less effective westward expansion, which is fine by me. I wish we'd failed the dream of Manifest Destiny, honestly. Would the interior have been left alone? Unlikely, but a more pluralistic North America will be less bad for the Indians. Indian Removal is less likely, and the Cherokee more likely to get their own state. (With slaves and everything, how civilized!).
Not sure I agree here. Most of the pressure for westward expansion was local. Manifest Destiny was a concept that united anglo-americans regardless of whether or not they were united in a strong centralized federal state. If anything, the effects on native Americans might be worse, since it was often the federal government or the federal courts that inhibited uncontrolled settlement and upheld Indian land claims. On the other hand, absent the federal government and the legal precedent of the Northwest Ordinance, the potential for conflicts between states as each furthered its own version of manifest destiny could create some interesting conflicts Something resembling OTL's Mexican War would be fairly unlikely, but I suspect that ultimately the less-united states of anglo-america would still find a way to expand to the Pacific at the expense of Mexico, either as one lose confederation or as several independent states.