I'm in favour of keeping Canada and Mexico in the game, but the size of the Neutral Army should probably be increased (particularly in Canada).
I like the way you're running the different game styles, but I'll wait and see what the various missions you chose were before commenting further on that. If Canada was to be split up, it should probably no longer be neutral but regionalised like the general map. (In such a scenario, it may be best to play without Mexico at all).
I think the regional reinforcement bonus for the New England and Southeast are one too high, given their levels of defensiveness. For the same reason the Plains is probably one too low.
Anyway, you've done a great job running the games and I certainly won't complain if no changes whatsoever are made.
And there's a variant, not covered by the rules, where everybody gets deployments and then everybody gets to attack, and the starting player of these two phases rotates around the table, which I played for awhile. Also works well.
This sounds similar to the variant released on the 'Risk II' computer game called 'Same-Time Risk' (which is the prefered rule format when I play with friends). It allows all players to issue orders simultaneously - therefore eliminating the significant disadvantage of being the last player to get a turn.