I think its fair to say that an earlier opening of Japan would lead to earlier Japanese Imperialism.
As Japan moved into China, the likelihood of a Chinese nationalist resistance is slim, as nationalism was simply not part of Chinese philosophy back then. I think a more appropriate model would be to think about what happened when China was taken over by the Mongols and others, who were considerably non-Sinic peoples, but pragmatically accepted as Huangdi. A similar thing could happen with Japan...
1626: The Manchus begin their military struggles with the Han Chinese, at first the conflict starts as a rebellion against the Huangdi's policy of extracted tributes from neighbouring states, but it soon becomes evident that certain Manchu aristocrats have higher aims.
1635: The Seclusion Laws of Japan ban incoming traders from anywhere other than China or the Netherlands.
1637: A Jesuite inspired tax revolut occurs in Japan. In the aftermath, 7,000 rebels are executed, and a further 30,000 forced into exile (POD). Many settle in Korea.
1644: The last Ming Emperor hangs himself as the rebel Li Zicheng sacks Imperial Beijing. Imperial forces stationed near the Great Wall attempt to restore order, but any march away from their camp would have surely been taken as a threat by the Manchus present all around them. General Wu, not wanting to have to face two enemies simultaneously suplicated himself to the Manchus and invited them to take Beijing.
1644-64: The first Manchu Qing Emperor, six-year old Shunzhi is crowned, but Ming loyalists continue to roam the empire, raiding villages and attacking Government officials, the mountains of north Korea (which unlike our time line were never held by the Manchus due to (among many other things) persistant skirmishes with the Christian Japanese settlers) became one of their strongholds.
1653: Following the success in exiling the 1637 rebels, it becomes official policy to exile Japanese Christians. Many of course end up in Korea, where they settle, occassionally integrate with the natives and preach. Over the next 50 years over 100,000 suspected Christians settle in Korea.
1651: A Japanese trading junk is set ablaze by Ming loyalists in the Korean harbour of Weonsan. Important Japanese traders begin to exploit feudal links to keep trained soldiers aboard their ships when sailing to dangerous regions.
1658: In the light of further instability in Korea, Japan sends mercenaries to eastern Korean harbours and towns to maintain a safe trading environment. The mercenaries protect Japanese ships but often turn their brutality towards native Koreans, every woman in these harbours have either been raped by a Japanese mercenary, or knows someone who has. To combat this, it is permitted for the mercenaries to settle in eastern Korea with their wives and families. As Japanese trickle in and native Koreans flee brutality a process of japonification takes place on the Korean east coast.
1660: A Dutch ship captained by a Pieter Alecsander Von Quadt arrives in Ch'eongjin, north-eastern Korea in March, where the local mercenary lords are fascinated with their strange ways and impressive tactics in securing ports (the Dutch tell tales of Europe, and how even its most backwater nations offer security to traders and merchants). Amicible relations are established and trade in curios trickles into existance.
1662: Captain Von Quadt returns to Ch'eongjin in July. He reacquaints himself with the mercenaries he made connections with two years ago, who later invite him to visit the Japanese homelands, and to talk to advisers of the Shogun. He eagerly accepts and Von Quadt becomes the first Westerner to visit the Japanese capital Kyoto.
In August, while Von Quadt and crew are in Kyoto, a party of Han Loyalists and Korean Refugees raid Ch'eongjin and do battle against the now heavily civilianised Japanese population. Many Japanese face the blade and the event is come to be known as the Ch'eongjin Massacre.
When word reaches Kyoto, Von Quadt is as outraged as the Japanese themselves. He oversteps his powers of suzeraignity and pledges to the Japanese government that the Dutch Republic would support any action that Japan wished to take. No physical support ever materialises but the act of pledging it helped cement Nippo-Dutch friendship.
1663: In the bleak winter a Japanese force of over six thousand men invades northern Korea (wearing furs (a few of which brought over by Von Quadt and his fellow Dutchmen)), and begin to shake the Loyalists out of their hiding places. Casualties are high, but ultimately more people are lost to the cold and mountaineering accidents that due to enemy engagement.
By the spring most of the Loyalists had been slain, as had nearly two thousand Japanese... At last the campaign came to a climax as the force discovered Zhu Yourang, Prince of Gui, and last Ming pretender to the Imperial thrown, hiding in a cave atop Mount Paektu. In a dramatic gesture, the Japanese force marches their prisoner to Beijing, where he is presented to the nine-year old Emperor. The sight of the battle-hardened Japanese General clothed in Moldovan bearskins presenting the traitor to the infantile Emperor and his Mongolic Dowager is a striking image to many Beijingers, the opinion begins to spread that perhaps the Mandate of Tian does not belong with the Manchus at all, but rather with the much more Confucianic people to the East.
1667: The first shipment of firearms reaches Japan. Shogun Tokugawa Ietsuna loosens the Seclusion Laws, effectively granting trading rights to European allies of the Netherlands. Pieter Alecsander Von Quadt (or Piite No Kowadda, as he becomes known locally) becomes the Ducth Ambassador to Japan.
1668: All is not well in Beijing; the establishment is thoroughly ashamed in not having been able to sort out the Ming Loyalist problem themselves, and confidence in the young Emperor Kang Xi is low. And so, the Huangdi's minister, Oboi, has the young emperor's grandmother and dowager assassinated so that he himself could obtain dictatorial powers. The thirteen year-old Kang Xi is helpless and is assigned to house arrest as Oboi attempts to reassert power from Beijing. This move splits the Manchus and scuffles begin to occur throughout Manchuria.
1670: The first Renaissance texts reach Japan, they are not widely translated or read, but they are indicative of an introduction of Western thought.
1671: The newly appointed Shogun, Tokugawa Nobuyoshi, a young, pro-Western man with a keen skill for shooting, decides that in the light of growing strife in Oboi's China, the time is ripe for a Japanese to claim the Mandate of Tian. And so, that spring, he assembles the most modern army East Asia had yet seen and marches toward Beijing.
During his campaign he co-ordinates anti-Manchu forces and when he eventually reaches Beijing he is welcomed by many of the city's citizens.
However, when he reaches the Forbidden Palace he performs a move that shocks his contemporaries. Instead of announcing that the Japanese Tenno is assuming the Chinese throne, he claims the throne for himself! Forming the Sino-Japanese Empire, with himself as the first Huangdi of the Dechuan dynasty.
1671-8: These represent difficult times for Japan. Overnight the Tenno has been made a vassal within his own empire. Nobuyoshi abolishes the Japanese imperial system upon his return (as well as the rump monarchy of Joseon Korea), and instead delegates authority directly to the aristocratic governers. He moves the Empire's capital to Gongju, Korea, to compromise between sitting in Interior China and Japan. He introduces religious reforms that no longer blindly discriminates against Christianity as a whole, but rather discriminates only against Catholicism. Finding that as Emperor of a ruge terrain there is nowhere left to reasonably place exiles, he instead institutes a policy of imprisoning and executing Catholics. It is believed that his motivation for doing this comes from one-sided research from badly translated European texts that associated Papal authority with inappropriate foreign imperialism.
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So there we go, a Sino-Japanese Empire, under the rule of an almost 'Modernist' Emperor. Tokugawa Nobuyoshi is charismatic and militarily brilliant, but would his empire survive him? It is possible that neither the Chinese nor the Japanese are ready to support his new order, as conservatism was a strong force in both societies. But perhaps when his modernising program becomes increasingly profitable to all involved people will forget why they ever doubted him?
Although perhaps the most interested thing I have left here before trailing off is the Emperor's involvement in European faith politics. His ignorant persecution of Catholics may come to haunt him, and it is indeed ignorant, one of Nobuyushi's greatest flaws is his refusal to consider the alternative to his argument, once he has decided Catholicism is a negative thing, he will only concentrate his readings on anti-Catholic material (and there was a lot of nasty anti-Catholic material around at the time), hell, I wouldn't be surprised if Nobuyoshi's sole knowledge of Judaism comes from 'the Protocols of Zion'...
Perhaps he is setting his Empire up for a conflict with a great Catholic power (probably France) in the future...