I'm worried that figure isn't going to survive this encounter, I guess the same applies to Sensei, but he did dig himself into this one.
He told a lie to save himself from embarrassment, sure.
Did that lie force his students to act like a bunch of possessive psychopaths? Not really.
The situation getting out of hands is on them, I'd say, cause they're the ones who are so pissed off at the idea of Sensei having a date they'll pull out the firearms, despite knowing damn well he's far less durable than they are.
It feels like we skipped over a part, like leading up to the wedding and such, but regardless, so happy we made it here
Would have been the ins and outs of a remote relationship with their individual careers going pretty normally without each other to play of off, followed by an uneventful (apart from being touching) wedding due to Harumi being too reliable when he's on to things. I think Shiromanta would prefer to not conjure up artificial drama for that interval.
Brellas are very close to shotguns in behavior, so I think a Splat Brella would be a better pick for her rather than a Blaster. But I guess Mr. Grizz doesn't care about giving the girls what they want.