random thoughts:
it seems to me the war would revolve around New Orleans... there are some other towns in the region, but that's the big one... the US really wants NO, and capturing it would be priority #1. Fighting in the rest of the territory would be tough, as it is literally a howling wilderness in most of it...
what exactly is going on in Europe if Napoleon is messing around in LA? It seems he would have to have made a peace of some sort, or he couldn't afford the movement of ships and troops there. And does Britain really want him to have control of the region?
US/UK alliance... possible, but there is one big stumbling block: British impressment of US sailors. Basically, so long as Napoleon is a threat to Britain, they are utterly dependent on their navy. Unfortunately, the RN was a hellish place to work, and the common sailors deserted in droves; the RN responded by stopping US ships and grabbing any English speaking sailor who couldn't prove he was American (a harder thing to do back then). Naturally, this infuriated a lot of people in the US. To have an alliance, you have to have Britain stopping this policy, which would mean that Napoleon is no longer a threat to Britain... so, again, just what is going on in Europe?
One positive thing the War of 1812 did was establish definite control over the Old Northwest, putting an end to British hopes for a big Indian buffer state and keeping the Americans from moving further west. What happens to that in this ATL?