Hey El-Phil beleive it or not it's good to hear from you
Anyway seriously on to your points
Point 1: The Nazi's are going to have had underground facilities to build their ships, remember this is early space age and starships of the era were "tiny".
And with modern stealth you can hide a surface ship from a satelite (Sea Shadow I beleive is the name of the new stealth ship) and there are plans for entire warships built around Sea Shadow tech.
Assuming the Icaranverse is a few centuries ahead of us their stealth is probobly pretty good.
Point about the UN fleet, you do realize how BIG a solar system is right? The Icarans though powerful were hardly everywhere and considering in this point in history it still took months for a ship to travel from planet to planet it would actually be pretty easy to hide a few dozen relatively small ships in the asteroid belt.
These are not the multi hundred ship kilometer+ sized ships we see later the Icaran navy was at best a hundred and fifty ships most of which were only a few hundred feet long.
Point 2: Well with secret police (I.e shadow daggers) and a government that is not above taking out entire rebellious elements and slaughtering them we are looking at a situation where it becomes a "better red then dead" situation. An iron fist hidden in a velvet glove, the Icarans would treat you as a full citizen if you complied or kill you if you didn't.
Look at Rome they were not forever at war with their newly added provinces, though they did deal with "minor" raids and border skirmishes throuhgout their history. No doubt Icara would deal with the same but that's not really worth mentioning with an Imperial scholar describing millenia past events.
Point 3: It was after both sides had been nuked and were getting their asses handed to them left and right (if you'll pardon my expression). It was simple survival they still did not much like or trust one another.
And after being shot into a foreign star system where the bulk of the populace landed on Icara (and were Icaran) the ESS holdouts still took a century to integrate and even afterwards we see a streak of defiance in that they have distinctly asian names and culture and also often refuse to intermarry with the "Icaran race". But sheer numbers is on the side of the Icarans as is economic strength and military power, and after heading into the stars once again they found aliens and other human empires that would be more then happy to watch them burn (or in the case of some aliens more then happy to EAT them).
So it is simple survival again, rebel against Icara maybe succeed but get eaten by an alien or absorbed by a human Empire who didn't much like your ancestors Or join Icara and live to reap the rewards.
Kind of like Russia and the US after the Soviet collapsed Russia still doesn't much LIKE us and we don't much like them but we kind of need one another for the moment.
And if you notice a little bit the Asian populace in a good part is still rather distinctive and it's own small "Empire" within the Icaran Empire. The bulk of Brandenburg, the entirety of Sparta and many other worlds are almost entirely "asiatic" in heritage. But after centuries the two cultures being forced to rely on one another have by and large merged into the single Icaran Empire.
Point 4: The Chinese have always been adept at scientific adaptation and development, hell the Japanese who were non-to bright in any comparison to the west or China figured out how to replicate firearms on their own in rather short order. The Chinese already had a basic understanding of gunpowder and rockets, and in fact were well on their way to producing firearms of their own almost in parallel to the west so if you introduce hundreds of captured Icaran weapons and I have no doubt a culture that felt threatened with it's survival would replicate it quickly.
As far as training large numbers of men the Chinese always did have the ability to raise vast armies (I beleive the Qin had over 400,000 men) and with muskets you don't much need training, which is why they were so great. Add to that the Yuan went on their own little bender of conquest and they would expand their own manpower base after a few decades.
I didn't say their replicated firearms were as good either, just copies but given Chinese population vs an invading army that had already been blooded in previous wars and overwhelming numbers of slightly inferior quality beat out Icaran tech.
Point 5: Thanks I'll fix that ASAP