whoever holds the forts is extremely important. If it were the French or Spanish holding the forts west of the appalachians, the settlers have absolutely no protection. militia from the British Colonies cannot cross over to help them out, and the French/Spanish aid the natives in driving out the settlers. That makes it difficult for the intruders to get a foothold. with the initial guys getting wiped out, fewer are going to follow.
Time only benefits the defenders. France could have held off invasion for quite a while, giving their slow growing populace time to grow. But that's irrelevant for this POD, as the French are kicked out. The only bone of contention is south of the Tennessee river, where no one was doing much of anything except the natives. East of the mississippi is a lot of buffer time for Louisiana to grow.
And there's still this assumption that the psychology of "USA = freedom/advantages". We don't know that. IF the British countries war with each other, turn to dictatorships, deteriorate, people may not have that mindset. Texas was no picnic, and was rescued by inclusion into the US. Folks might look at an ATL version of that, see failing efforts, and decide the grass is not greener on the other side. OTL kentucky debated at one time whether to join Mexico for protection. Had USA east of the appalachians been a jumble bumble, they might decided to pull a 'Texas' and abandon whatever US country they were attached to and join New Spain. OTL USA was one big improbable miracle. making changes to that perfect storm upsets the whole dynamic. We don't know that Louisiana is automatically going to pull a Texas. If Louisiana remains in Spanish hands, Texas may not pull a Texas.
The central/western part of OTL US is sparcely populated. It's going to take a long time before any part of it is going to be able to stand on its own. it has to look to a stronger region to be a part of. If the eastern seaboard is not strong enough to protect them both militarily and economically, they aren't going to break away.
The whole continent may balkanize, but I don't think it's fair to assume it automatically does because OTL Texas, west florida and California did (for a brief time). All three countries would have failed had there not been a strong USA to absorb and protect them.