I don't see why. If anything it is pretty stupid that the X-men don't get stuff like Magneto helmets or power dampeners or other tech mass produced. If such things existed in the real world then we would certainly try to produce more than just 1. Look at cryptocurrency and blockchain. Everyone trying to jump on the hot new wave yet you telling me in a world where people can control minds, no one has ever thought to mass produce telepathy blockers. So really the issue is the solution here, the issue is the comics are largely filled with people that are stupid. The MU should have people with a telepathy blocker app on their phone and ****.
Think you are missing the point. We agree she can kill them. You just said how much better Nova thinks she is than others so waiting a week doesn't really matter all that much to her. She is arrogant and cocky. She took the hostages and waited. Oh well.
You can create things that are one way. In fact bitcoin and blockchain technology are based on cryptography that is one way. So the helmet is a piece of tech. You can make it one way if you so chose. In addition, just like you have voice, facial, iris and fingerprinting software designed to just recognize a single person, if you can scan and pinpoint Jean's telepathic signal, it would be easy in a world like the MU to tailor a helmet that only receives her signal.
So really you simply want to presume these things because it suits your narrative. That is your prerogative but in a world where all sorts of impossible things happen, your argument here is just odd.