A couple small corrections, but overall great reaction.
1.) So Naobito was able to deal with the domain essentially by upping his reaction time and dealing with the fish automatically. Nanami was telling Maki her only hope was to try to attack as soon as she felt them similar to what Naobito was doing because she doesnt have cursed energy to reinforce her body as a defense boost like he was going to do.
2.) Megumi did use his domain in season 1. It was named Chimera Shadow Garden. It was incomplete at the time. It spat his shadow out across the entire floor essentially allowing him to use its pocket dimension properties over the entire field as well as create multiple copies of both his Shikigami but also himself.