since you want to be a simpleton
an easy explanation
1. zoro always had the willpower to use it but used it the wrong way, by regulating his haki
2. kid oden didnt have better ryou than current zoro. Its not about how great your ryou is . The sword exudes your haki on its own.
3. regular people get drained off the bat, people with stronger will powers will continue to fight even with their haki being drained.