you seem to be asking (at least) two separate questions here, so i'll answer both of them. first, a lightsaber would cauterize even a bisection, but the shock would still kill you. as mentioned above, the heat required for that would never work in the real world, but these are frakkin' lightsabers we're talking about. nothing about lightsabers would work in the real world.
second, lightsabers don't cut through everything. even in-universe, lightsabers are stopped by a material called cortosis. by extension, we could imagine that adamantium might have similar properties.