Maybe you are right too, the reports about how Japanese soldiers marched solemnly to various temples and shrines they took after battle, while offering respect to their shared Eastern Gods and Buddhas are definitely signs that the Japanese seen their conquest as a Holy War to keep Middle Kingdom from being fallen into Western hands. It may sounds superstitions to Westerners, but the various Jizos erected on their campaigns definitely help their morale. Especially with medical center and food supplies given for often illiterate and starving Chinese Peasants. It was either the perfect Propaganda techniques or borne of genuine spiritual desire. So much that in late 1920's majority of "Japanese Army" operating in China are made from Chinese volunteers working under Japanese officers.
I think it was both. For Japanese government leaders and higher-ranking officers it was primarily the former, though even among them in many cases - especially after the Showa Emperor had taken up residence in Hokkyo/Beijing - it was of the latter, out of a sense of awe (?) or conviction that the Emperor of Japan was now also the Son of Heaven. Among the rank and file and junior officers, it was usually the latter.
With regard to helping the populace though, I'd connect it with the ones below...
Maybe have Japanese army do the treatment reserved to Western Missionaries and Christians IOTL enacted to all Chinese and maybe Koreans too. Instead of just killing Missionaries and forcing Christians to convert or die... maybe they do wholesale slaughter of Chinese villages and cities? Instead of just killing Christians that refused conversion to Eastern religion?
The convert-or-die attitudes to Christians however, is also the one who made American Fascists having love-hate relationship with the Japanese Empire. In one hand, they actively persecute the coreligionists of the fascists, in the other hand, it was well known that Umago Nakayama and Sho Nakatadashi are American style Christians and very unsubtle puppets of the pre civil war US Governments. The US sticking their nose into East Asia too much is often cited as the reasons of the Great Depression and subsequent Civil War.
While there were massacres and forced conversions of Christians and missionaries, it was never officially sanctioned, either before or after the assumption of Mandate of Heaven. This was usually the case only with overzealous office Han or Japanese officers, either out of their own xenophobic conviction or influenced by their subordinates, and was always strictly punished. When Japan modernized, and in particular their army one of the things they learned from the Prussians was an appreciation for iron discipline. Troops going out of control was seen as counter to this principle, and when the Mandate of Heaven was assumed...
...well, remember that much of China's destitute state was due to the sheer corruption of both the the republicans and the Qing at the end of their dynasty. Roving bands of soldiers (in name) would loot, steal, and rape their own people, because their officers didn't care or had embezzled the funds needed to feed the troops. Ditto for the civil service, with bureaucrats more interested in enriching themselves and fawning over their superiors and the wealthy while doing only a facade of their duties.
For the Prussian/German-influenced Japanese, the utter mess that were the Qing Armies and later on the so-called 'National Revolutionary Army' were a disgrace, not worth being called a military or even soldiers at all, while the highly-organized Japanese civil service were appalled, especially when one remembers that in China, a functioning civil service was seen as characteristic of a prosperous dynasty secured by the Mandate of Heaven. So when they assumed the Mandate of Heaven (or even before then given the number of corps and field armies the IJA had in northern China maintaining order and largely composed as you said of native volunteer regiments commanded by Japanese officers), they made damn sure out of sense of pride to get proper government and authority back up and running.
I'm not saying the Yamato and the majority of either their Han or Japanese subjects are big fans of Christianity (they aren't), but in the interests of a fair and globally-oriented society and nation, Christianity and other religions are tolerated along with the major ones...though it's preferred they don't meddle in politics. It might be hypocritical, what with Yamato legitimacy anchored in both the Mandate of Heaven
and their divine descent from the gods of the Japanese Home Islands, but hey, western politicians swear oaths of office on bibles. Is it really that different?