Im actually worried about this
No matter how bad they are, shoot first ask questions later is honestly a horrible thing, and your joke about how we're using iowas ow tactics is, frankly a bit on thre nose in terms of why I have issues with the idea of a preemptive strike
Yes, it is definitely morally slippery. And we should probably remind Mami and Sayaka about that in our response.
I'm actually a bit surprised at how OK they all were with it in this update (well not Homura, her enthusiasm is perfectly expected). I suspect this may be the result of Mami's dependence and her being afraid to counter our proposals and as for Sayaka-- actually, now I think about it I can see Sayaka just taking Iowa's rep and our description of them at face value and assuming they must definitely be evil. So maybe that's not so out-of-character for her.
But despite the jokes, a pre-emptive attack before we even know whether they really intend to come at us won't be morally white, it would be morally grey at best.
The problem (and the reason it's not morally black as well as why I'm not completely against the idea) is: their information source.
If their reputation for decapitation strikes is true and they have enough info to work with, they could feasibly come up with one that works. It's nothing to worry about if their source is just a far-seeing clairvoyant like Sayaka thinks it is (like Matsuri Hinata from Suzune Magica). We're in trouble if it turns out to be something more exotic, like a mind-reader or a post-cog or, god forbid, Kyubey himself. Depending on what/how powerful their information source is, they might be able to drop the Potentialbomb on Homura before we're ready to deal with it or Witchbomb Mami. Basically, they could be another nightmare headache like Oriko. Now that Oriko herself no longer seems to be a primary antagonist, it's plenty possible Firn came up with somebody else to fill the same role.
If this is the case, the very act of approaching them to talk could backfire badly and if they plan to attack, depending on how much information they can pump from their source, the longer we leave them, the messier dealing with the whole thing could become.
Our cleanest option really is just to conduct a mass kidnapping.
In fact, if we do it right, it's possible to make it extremely clean, to the point
the victims themselves never notice.
Men-in-Black style.
Grab all their Soul Gems simultaneously under Timestop, wake them up separately for interrogation one at-a-time with Antimagic restraints on under Timestop. Keep their heads in a Privacy Field so they don't see or hear anything while we X-File their brains and read through each of their memories. Afterward, we can wipe all their memories of the encounter and dump them back where we found them.
All done under Timestop. Zero time will have passed for them. Zero memory of meeting us. For them, the whole kidnapping will never have happened.
It's possible to make it so that nobody but the people carried it out ever knew the operation even happened. No harm, no foul.
It would be really, really morally grey. But possible.