But If Britain has Naval Dominance than the US wouldn't be able to send ships to the British Isles to launch the chemical weapons thus the British won't have to worry about American retaliation (except for in Canada although that would be nowhere near as devastating to the British as losing the east coast would be to the Americans) also I never said Canada could hold out for the whole war, only that they could hold out for a while and would kill large amounts of American troops which is entirely possible especially if you factor in winter and the attrition effects it would have on the American Army. As for the 20 miles maximum battleship range I concede I did not know that but in the end the same population and manpower devastating effects could still be put in effect with even more chemical weapons as well as other longer range explosives. I also concede that blitzkrieg may have been an overstatementment but A British advance to the Appalachians could still move quite quickly considering the entire area between the and the coast would be devoid of human life other than British soldiers. Finally the armed civilians would not be an issue since the British with my invasion plan wouldn't be occupying a any cities but would be DESTROYING them and repopulating them with British people after the war.Chemical weapons on cities? No that would never be done because it invites the same retaliation in kind and WW1. And no Canada could not hold out. Britian realized and accepted that in event of another war with the U.S.A after the War of 1812 Canada would be lost. This is why she didn't fight another war with the USA in North America.
Also American manpower won't be destroyed because the range of battleships is only effective as 20 miles maximum effective range. Most of the population doesn't live within 20 miles of the coast because of that nice vast interior.
Blitzkrieg probably won't happen because 1) The British stopped all armor experiments about armored exploitation which was picked up by Germany and 2) Their WW1 experience generally wouldn't allow from it as they drew different lessons then the Germans.
You clearly don't understand Americans. To quote admiral Yamato "You cannot invade America. There will be a rifle behind every blade of grass." As such in addittion too the regular army now you got the good old american guerilla to contend with. Good luck with stopping that.
Finally Japan would not be called on for help. Western Powers were quite content on keeping everything as it is and not expanding anyones power. Especially Japan who looks hungrily down upon the East Indies.