But it takes far greater will power to break/abandon such principles
LMAO no man, that's the opposite. Your principles are governed by your will so it doesn't take any willpower to go against your will.
If Zoro didn't have the strength of will to listen to Luffy and endure that ass kicking in Jaya, he'd have fought back and they ultimately would have lost even though it meant avoiding the short term L
If Nami quickly denied Luffy's claim when Ulti was beating her ass, it would have taken zero willpower to avoid that uppercase L
Sanji resisting against all pain and betting on things to still work out for the best while not improving the odds himself, it's his force of will. Right or wrong