1. Last I remember, the biggest losses to an opponent's defenses were sending a few units under their break and failing intentionally near their break, however, this method doesn't cripple their country, only the MUs you're targeting. In saying that, one should note that military strategy tech exists, tyranny govt has an attack gains bonus, etc. Another primary factor in defenders losses is how similar you are in NW. Assuming no tech, sending the minimum before your generals revolt will take 50 hits to cut a break in half with fails = sending 11 breaking hits = sending 9 just barely not breaking.

2. Just under 1/3rd of your units will do 4% on Tyr and 1% on the others

3. Likely you were closer in NW, had more tech or had an attackers govt like Tyr.