Concerning the AI invasion thing:

I only play on m/h with the HRE....invading Hungary at that point in time....and guess who knocked on my backdoor? The Moors & a full stacked army led by their heir landing on my [pretty much unprotected] Italian shores. Thank god I had an army in Souther France [which was about to invade Spain] to defend Rome.....

I think the AI on the map ain´t that bad.