Huh, I just ran the first turn to test it and it seems like if the first turn is good weather, you can move past the fort with no cavalry guard and if it is bad weather it blocks you.
Interesting, I didn't know that weather has an effect on zone of control.
The Austrians can send an army to the same province in the first turn that is true but if you send most of your army there, you're going to be happy about it because you have a nice first victory
The Austrian army is not your equal at the start. If they attack you, be happy.