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. Around 1/3rd of your units will do minimum drops 4% on Tyr and 1% on the other govts. The graph for determining it looks like a hockey stick tho. If you're going to spam hits down to <25% readiness, it's best to be hitting with around 25% of your units to be readiness efficient.

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