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 (a lot in this game do). The closer you get to sending all of your units, the more devastating the impact to readiness on a curve that goes all the way to 3 hits and you're done. 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 because if you start by sending 1/3rd you won't end that way.

3. Likely you were closer in NW, had more tech or had an attackers govt like Tyr. There's absolutely a reason for it if you combine your govt bonuses with your NW differential and tech.