I'm pretty sure that at this point he's demonstrated what he needed to demonstrate. Also pretty sure she doesn't care about that.
Yep.
He's doing it wrong. He's showing off techniques in a battle that ultimately doesn't matter, which means those techniques won't work in the upcoming battle against the actual villains.
Honestly the only way this obligatory show off becomes redeemable imo, is if the Elders who insisted on it and/or questioned the validity of Arc's abilities, literally start worshipping Arc like a god. Not just treat him with respect, but actually become willing to kneel before him and be at his beck and call.