Another thing I'm having problems with is the ahistorical results.
I think in any game you can come up with this sort of argument. The question is, given the type of game, is it feasible for this to happen at all historically? I would argue that it was possible but not probable for the british to land and march to an undefended paris.
Also, anytime you involve a computer AI that isn't scripted or a human, you're going to get ahistorical results. I don't know about you, but I personally haven't met a wargame yet, either on the board or on the computer where I haven't heard a charge of 'this game produces a-historical results'... I think what it comes down to is the individual's tolerance level for their own brand of acceptable abstractions.
Likewise the Austrians hunker down in the Tyrol massing huge amounts of troops there knowing that the french can't fight them with their strength in the mountains. Then they blitz Switzerland and push units through the soft underbelly of France and on to Paris in a couple of months. Even moving the entire Army of Italy into Switzerland cannot prevent this and it happens even if Napoleon is besieging Vienna. Oh, and they have dropped a line of depots ahead of their attack, inside france, so there is no surprise even as to where they are going.
I used to encounter the same problems with france after my first few games. Now the austrians usually get pummeled by me in either switerzland or tyrol and they all head back to styria as I march my armies straight for it. It's a function of how the AI works... they will try to push for your capital or defend it's own. If you occupy his capital first, then it's more than likely he will send his units back to defend it asap. If not, the AI is generally more aggressive and will push to your capital. It reminds me of the austrians in the 1796 italian campaign where they launched 4 offensives and each time were driven off, even though they took higher casualities over and over. After arcola, they scuried back to tyrol since they were sensitive to that area.
Defeated units run amok behind the lines. There seems to be no rules encouraging broken units to retreat toward supplies. Even if I can defend Paris and decisively defeat the English and Austrian armies in France, I know that I will then be forced to spend the next 6 months using disproportionately large numbers of troops trying to put an end to these forces.
Try a different strategy...
Surround the attacked province(il defrance) with single unit division/militia and what you'll usually get is the defenders to surrender. I will usually pull them out of the cities and this does the trick for me more often than not. If I have to fight the retreated units, then I use the call reinforcements button.