Normally producing any high level seals take a huge amount of investment. To get a ninja to sealing rank 60 takes roughly 4000 exp. so 4 years of an above average ninjas life. On top of that sealing is incredibly dangerous. Let's say half of all new trainees die before they make anything that is useful. Then you have the actual cost associated with building labs and safety features
The Kurosawa clan can skip all of these hurdles thanks to the Iron Nerve. They only need one top level seal master. Each member of the Kurosawa Clan can crank out 100 seals every day. This cost nothing. They spend no extra experience. They aren't risking there life to seal failures. They don't require safety features or labs to do research. All they need is ink and paper. If Ren can convince 10 members of the clan to join her conspiracy every day she can equip every Mist ninja with the best seal ever designed by a Kurosawa seal master. She can repeat this day. Moving on from one seal to another. Each seal giving every Mist ninja more power and utility. Every day mist is growing stronger than every village. She has a cease fire in place. Which is all she needs to become powerful enough to fight of all the other villages
Assuming, of course, that she's the kind of person to do that in the first place. Mizukage or no, she's a Kurosawa and, if her favour among her clan is any indication, a wholehearted believer in the clan ideology. That she might not even
want to set up that kind of enterprise, seeing it as an affront to the Kurosawa, would not come as a surprise to me. It's also possible, as you said, that she
is doing it, but I wouldn't talk about it as if there's no way she's
not doing it.
I just binged on reader mode, so is that discussed explicitly in a rules clarification that's not threadmarked? It would be surprising to me, considering Hazō's sealing failures.
An Iron Nerve user can, upon seeing an infused seal, perfectly replicate it later, and if they've drawn a seal once they can also perfectly replicate it later. Typically a sealmaster will only infuse seals they personally drew because sealing failures can be fatal and sealmasters aren't going to trust that some random schmuck copied the seal exactly right (since even the slightest of mistakes can mean instant death), but a Kurosawa can provide 100% guarantee perfect seal blanks. This is a great benefit because sealmasters often can't or don't spend all day scribing top-tier seals (Jiraiya's constantly busy, for example) so a Kurosawa genin can spend hours a day scribing top-tier seal blanks at no time cost for the sealmaster (since infusing takes negligible time). We've seen this in action from Hazou a couple times, though the most clear memories of such events I have pertain to Hazou offering that ability to Jiraiya, who as a normal sealmaster was very creeped out at the idea of infusing seals he didn't draw as a matter of reflex.
However, the Iron Nerve only guarantees the seal blank, and it's still quite possible to fail the infusion roll, which is where Hazou's seal failures come from, so the Iron Nerve isn't a
total OP seal-machine. That said, in a world where seal output is presumably limited by manpower, proper application of the Iron Nerve could, as Oneiros has been arguing, easily have world-changing effects.