I assumed it was because none of them knew how to hit Harry, and Potter just wanted to watch a lightsaber duel. If any individual Sunny had tried to hit him with a Somnium, they knew it would probably fail, and Harry would probably curse them.

Also, I'm not sure how it was meant, but Daphne seemed more like she was under a literal spell than a figurative one, given that her crush reached sanity-compromising levels so quickly.

Sanity-compromising? Because she cast a complicated spell during a play-fight?