[Edi]

1) Oblique missile fire hits less often because the distance is greater. Instead of using the Pythagoras equation for calculating real distance (a^2+b^2=c^2 and then deriving c), the distance is calculated as a+b, so it's much longer. Hence less accurate.

2) Army movement happens according to nation number, from what I've seen. Nation with lowest number goes first, so they always move before you do. This is not as it should be, but I suspect fixing it at this point would be far more work than it is worth.

WRT the AI moving, you need two armies capable of crushing it. You chase after the AI with one and use the other to hit the province it will go next, which will result in it being defeated.

3) Moving in from two different provinces is certainly possible and often works. I have no idea why you've had such bad luck with it. A debug output of the game log could help sort that out, but you'd probably need one of the more experienced debug log interpreters to look at it for you. I've less expertise on that front.