Komamura did not stop it with one hand. He blocked it with his armored arm pad and helmet, the latter of which got broken. You're also making a double standard when it comes to their personality. You said "if" Tosen were bloodlusted or Komamura were serious. The same applies to Zaraki. It's fact that he limited himself in character two fold, one by not using kendo, and the other having his reiatsu suppressed on top of not dodging or even trying to dodge attacks. It's a double standard. It's too bad Tosen wasn't bloodlusted, but he was still more serious than Zaraki. While he might have dragged on the fight on purpose, he was still using his best techniques and full power. Zaraki wasn't. He didn't even intend to kill. He let Tosen live until he scoffed at the mercy, then he was going to kill him, but Komamura prevented it.