Interesting discussion here... I don't have much to add, but I'll take a crack at it.
Japan is not especially rich in the resources you need to keep an army together - rubber, oil, steel - all useful stuff. A lot of their campaigns around Indonesia were designed to snag said resources, I believe. If they'd snagged their oil and other good stuff, then an attack on the US would be reasonable.
As for Japanese small arms tech, it worked on a lot of the folks they met with. A fair number of dead Chinese, Americans, and British wouldn't call the things underpowered. Sure, they could've been better, but they did work.
A serious invasion of the US? I think China would've been an obstacle to that - you don't invade England when Russia's knocking at your door. Lot of people live in China, and some of 'em were armed.
If one had taken place, I don't think that the average gun owner would've been more than a pest to an invading army. In the early days of the Revolutionary War, the militias had a bad habit of heading for the hills when faced with volley fire from the Brits. Guerilla tactics served irregular American forces much better. Take a dozen hunters with their deer rifles. Or a hundred. See how many would stay after the Japanese brought a few MGs to bear. Or after the aircraft strafed 'em.