That's... a possibility. It also includes weapons powered by impurity. Purity is really just the absence of impurity. Divine creatures banish impurity, while kami are pure spirits that can be vulnerable to impurity. So if you're going to compare it, the difference between magical weapons powered by "pure" magic, i.e the magic swords, are like boats that are rowed, while a blade powered by impurity is like a steam boat since it has much more power. The three years is because of just how reluctant your experts would be to work on it. The actual research isn't that hard.
Also IIRC, the issue with the kami and impurity isn't that the god is HURT by impurity, but corrupted by it. That's how you get rampaging gods, and why shrines are warded off from the outside world by barriers against impurity.
We got a few options here.
Humans with Impure power - Very risky, but fast powerup in the form of sorcerors.
Malicious magic equipment - Somewhat risky, since it'd tend to contaminate the wielder or else turn against them, but more containable if we had warrior monks(who as buddhists, are more resistant to impurity than shintoists, and also have the buddhist virtues to strengthen them against corruption). Where Shinto mythology has impurity banished for fear of tainting the pure, Buddhist mythology makes a big thing of taming the impure and putting them to good work protecting the faith.
Enlightenment powers - Slow to get, but we already have some in the form of our monks. Takes a lot of time to get significant powers though.
Divine possession - Something for our shinto priests and miko, taking the risk of allowing a friendly god to possess them and act through them. Risky to both god and host though, since aside from the blessings they're still easily wounded human and easily tainted god. Great for support roles, not so great for frontline combat.
Taoist sorcery/alchemy - Aka Omyoudo, Taoism lost the religious hold on Japan, but some of the practices stuck around in the form of Yamabushi and similar types. Not exactly impure sorcery, but wielding the powers of the natural world. We're using some of this in the form of talismans, but much much more could be done...if we could find someone who knows how.
Youkai corps - We got a lead into that with the ghost ships. Basically, they're ALREADY impure, and horribly violent, but some of them can be reasoned with, and bribed/recruited. Let the youkai handle the dark arts and powers. Though feeding them enough carnage may make them less reasonable as they grow strong.
So personally, a prerequisite to opening an Impurity division is to have youkai working for us(we got foxes) and buddhist monks to handle the intermediary work(getting them). Minimize humans and kami directly using those
stormtroopers
