Yeah, no. You can't just handwave Japanese and German somehow gaining "dominance in their respective oceans"; neither came close (and adding the UK and France wouldn't change things; the Allies didn't gain that level of dominance against Japan until 1944/45, and that was against an enemy that had basically run out of fuel and pilots, things the US would never have a shortage of). The US never really let its navy slack like it did the army, had almost as much industry as the rest of the world combined, and even if we somehow allow the US to lose a massive series of campaigns such that their fleet is knocked out, that will take time. Especially since, in the worst case, the US can always pull back its fleet to the mainland, and maintain a fleet-in-being that can't be easily sunk. Meanwhile, while also this fighting and attrition is taking place, the US is mobilizing its forces and fortifying.
It is true that the US was not mobilized for war in the 20's and 30's, but it was not as helpless as you presume. The inter-war US Army (after 1921) consisted of 12 Regular Army ("RA") infantry divisions, 18 National Guard ("NG") infantry divisions, and 27 Organized Reserve ("OR") infantry divisions. That is a total of 57 infantry divisions that could have been fielded by the US Army on rather short notice. The US infantry divisions were rather large. They were square, with two infantry brigades made up of four infantry regiments and an artillery brigade with about 200 guns (mostly 75mm but including a regiment of 155 mm guns), each division consisting of about 20,000 men each. The inter-war US Army also included 2 RA cavalry divisions, 4 NG cavalry divisions, and 6 OR cavalry divisions. So, 69 divisions all told. Then that coastline was bristling 27 RA coast defense commands. (See, U.S. Army Order of Battle 1919-1941, Lt. Col. Steven E. Clay.) After that, there were all the nutters with guns from Texas and other wild American places you just don't want to mess with. That's what you'd face, assuming you launched a successful sneak attack early in the game's time frame before the US can ramp up for war, and assuming you can defeat the US Navy, land an army on US shores, and supply that army over submarine infested waters. That's the best case scenario. But if the US gets even a hint of what's up your sleeve, its all over. For comparison purposes, after plenty of time to ramp up for war, with only a channel to cross, and with a cooperative host French government assisting with logistics, the British in 1940 managed to land the BEF in France that consisted of what? 10 divisions and three brigades? Sorry, pants down or not, it ain't happening.
Britain gave a solution to guerrilla warfare during the Boer war. How much will you be able to do behind barbed wire?
No man - no problem. If every family knows, that if they don't behave - one of their loved ones WILL die, it changes the perspectives. Will you go around, if you know that your daughter is subject to execution for your actions against? Just saying. Scorched earth policy works too. If you don't have gunpowder, I care not if you can make bullets - they are useless. Trust me, less than 0.1% of the americans will be actively resisting if occupied (unless part of the army). Passive resistance, aid to those who do raise up in arms - by all means. Easy to talk about it, hard to do if that means abandoning your family.
And those people also don't have a tradition of self-sacrifice and enduring of hardships. That's a hard truth.
Any local chemist can make gunpowder. If you know how to make nitroglycerin and gun-cotton you're 2/3's of the way there. This is not too far from the age of the railroad meaning lots of chemists who made nitroglycerin all the time.
