For most of Indonesian, monarchism might not be popular. But the OTL Melayu are still generally supportive to their sultan, and I don't think it could be more different in this story as well. Unlike those on the D.E.I, the sultan's at former British Malaya and Brunei at those day are a strong patron of Malay advancement in education, literature, economy etc due to the deliberate British policy of subtly preventing the locals from participating in local economy.
In fact, the reason why the Brits preferred the sultan's to keep their title is precisely to avoid provoking the Melayu in the first place. Malayan Union is the best example of this, to the point that even much of the pro-Indonesian Melayu leftist even went up in arms along the monarchist when they find out that the Brits is essentially threatening the sultan's with dethronement for refusing to sign a paper that essentially guaranteed a vast reduction of the sultan's authority that led to the idea being scrapped.
You should keep this at your mind when you planning to write your next chapter, especially during their eventual clash with the Javanese. Bonus when you consider that the Javanese are ardent republican in both OTL and in-story
Nice view you've got. That certainly broaden my knowledge as well. Don't worry, I will take notice on this.