Honestly, argument between two people unable to hold rational conversation is unsurprisingly uninteresting.
None of them is debating - they just pour emotions.
Ah, so in a tournament where main source of cheating is support magic the one who nullifies support magic naturally is at advantage. Clever.
+
I wonder if Naofumi bet on himself or...
1) It's not because he "doesn't want to" but "why he doesn't want to". Basically "he likes her not as sister" kind of deal.
2) He was beaten for trampling over her feelings. Without even realising it.