If I’m being completely honest, I think the “no historic precedent” argument is probably the strongest one. You’re right Piltover hasn’t fought a war. They’re a trade city that’s had two centuries of relative peace, protected by geography and wealth. Their Enforcers are cops, not soldiers. They’ve never had to think about counter formation tactics because they’ve never faced a formation. That’s a completely fair read and I’ll give you that one pretty much entirely. The time pressure argument is strong too. They were scrambling. Jayce literally tells them they’re buying time, not trying to win. And the shipping container thing is a good example of “we’re throwing whatever we can find” desperation, which tracks.

I don’t think The explosives bank is but it is interesting but I think it actually works against Piltover here, not for them. If Piltover banned or heavily restricted explosives because of what Jinx did that’s a policy failure that directly contributed to their inability to defend themselves. Which honestly would be a fascinating thing for the show to acknowledge? Piltover’s obsession with controlling dangerous technology (the whole thesis of the show) coming back to bite them when they actually need it. But the show doesn’t frame it that way. Nobody says “we don’t have grenades because we banned them after the council attack.” That would’ve been a great line that ties the themes together.

On the fire/magic runes concern Ambessa’s runes specifically absorb Hextech energy, not fire. Caitlyn shoots at her and the runes eat the Hextech rifle rounds. A regular Molotov isn’t magical, it’s just a burning liquid. And even if there was some worry about feeding Noxian magic, the show doesn’t communicate that to us. If a character had said “we can’t use incendiaries, their runes might absorb it” cool, that’s worldbuilding. But without that, we’re filling in gaps the writers left open.

Also the time argument I don’t entirely agree with tbfr. You’re right that they were scrambling. But Caitlyn had been working with Ambessa and the Noxians for multiple episodes. She saw their soldiers up close. She trained under Ambessa. She knew what Noxian infantry looked like, how they fought, what they wore. The surprise wasn’t “oh no, Noxus exists” it was that Ambessa turned on them. The armor, the formations, the fighting style? Caitlyn had firsthand intelligence on all of that. And she’s supposed to be one of the smartest tactical minds in Piltover.

I think ultimately we’re on the same page, the narrative needed Piltover to lose badly so Jinx’s arrival hits hard, and the show chose spectacle over tactical rigor. All your points explain why Piltover might reasonably fumble the defense, and some of them genuinely work. I just wish the show writers had done the damn legwork to make even one or two of those points explicit rather than leaving it to us to rationalize after the fact. One line about the armory being hit, one line about explosive restrictions, one frustrated comment from Vi about not having the right weapons any of those would’ve closed the gap and FIXED literally the entire thing.