Just make this beta sob actually tickle her. The set up for teaching the smug brat a ticklish lesson is perfect. Spineless paisen doesn't even have to push it out of his own volition if that's too ooc, maybe she just panics and clamps her arms down and he gets stuck wiggling his fingers under there until Nagatoro has a meltdown, it's a classic trope. A good minute of her over the top reactions as she gradually falls apart, apologizes and eventually just starts begging for mercy and this would've been absolutely top tier, up there with the Fujiko scene, a legitimate awakening-provider for future generations. I still haven't recovered from the blue balls, as you can probably tell.
Oh well. It's all in all okay, nice pit shots, anticipation, implications of Nagatoro being STUPIDLY ticklish... It's just kind of a bummer about the lack of pay off man. That's what fanart is for I suppose.