Looking over Stompy's protoplan...
No shot this works, right? Like, Kei got upset at how WMD-y our Storm rune was and by our own admission we didn't even think it'd be that bad. Right as a sudden impending deadline strikes us we go off to do an unexplained super-destructive rune that we hadn't seen fit to do before yet also don't think will take so long to interfere with the deadline. She's gonna figure it out, or Mari if she doesn't, or she'd just start worrying from the simple fact that a rune Hazou expects to be especially destructive is also likely to be a WMD.
In short, I don't think we can fool them here, and we probably shouldn't even try.
Speaking of, even though this is only one day of waiting, I'm a bit leery about it. How is this more to our advantage than meeting Oro ASAP and then taking a day to research if he confirms we have that much time? It feels like it's hedging against Orochimaru being stupid and pushing us to attack earlier even though we can spare a day to do the research. In other scenarios (we don't have a spare day, Oro sees the merit in letting us research for a day, etc.) it seems like a pretty straight loss.
(Also, I feel like we shouldn't be committing ourselves to Superchillers until we've talked with Oro and know what our odds look like. I'm of the opinion that we should only go for them if our odds look bad without, which we cannot determine beforehand.)
Edit: hmmm, looking back from the end of the plan, it seems like the intention here is to only get halfway through the research and then put it on ice until and unless we need it? I suppose that's fair, though we run the risk of accidentally completing it in one go. And I guess, if we imagine we can't move out immediately since it'd take a day to put our affairs in order (i.e. the rest of the plan), it doesn't really cost us time. If that's what you're going for, I think I can agree with it.
I'm uncertain that we should do a test summon here. Not wholly against, given that the travel time to Rice and then O'Uzu probably leaves enough time to recover with good treatment, but I'm not certain. If the numbers work out, it should be fine, but if doing a test summon now would risk delaying the assault in worlds where it turns out we don't have any spare days, we might want to hold off.
In a similar band of worrying, "when we know more in a few days" might give the wrong impression in worlds where, a few days from now, we're already at O'Uzu and about to fight. I wouldn't want to have Cannai falsely expect to have more time than he does to be ready for the battle.
What, exactly, is this referring to? The radar rune? Just the rift runes? This could probably be clearer as something like "All of our plans are shaky without further inventions to synergize with", which also feels like it casts Hazou in a bit of a better light.
Does this need to be said? I'm not sure what practical consideration it's really referring to. Is it "Hazou can't just YOLO a bunch of his RER2 runes because he might run out of FP halfway"? If so, pointing out our lack of substrate doesn't seem super salient, and we could reframe the line a bit better as "We still need to be careful when crafting our best runes, so it might take some time" or somesuch.
I agree with the spirit, but, no shot this works, right? The being vague part, to be clear. Oro's going to very reasonably need to know the specs of our weapon, what kind of devastation it'll deliver and how to plan around its deployment. We won't be able to keep to a "maximally vague" standard, and odds are if he does Intimidate us into giving up secrets we'll end up giving him enough pieces to figure out the insights we're trying to keep from him.
That is to say, if we want to actually keep the secret we'll need a more precise secrecy plan. A way to convey enough about it to satisfy without giving him what he needs to understand and replicate it. I don't off the top of my head know what that should look like, but I certainly don't think Hazoupilot is up to the task.
But overall, taking into account these criticisms, I like the general thrust of the plan. I would also like to add in a line about splitting the party so we can have quick comms with Leaf, but I think this covers all the bases we need it to.