The Sultans of Brunei and Sulu are neighbors and might move into political vacuums. Sulu especially, especially in Mindanao.
I see; nevertheless, even though Novales and the rebels won in TTL, they still confront the problems of the Philippines at that time.
And I see a problem; it was 1823, and most Filipinos are still consider themselves as members of their ethnolinguistic groups. Also, the surnames issue.
Just because Luzon falls doesn't necessarily mean the other islands fall. At the very least it would take them a few years to acquire the navy necessary to take over Palawan and Mindanao. My feeling is that Spain might eventually reconquer Luzon so long as they keep hold of the Southern islands- if Spain tried that in the Americas the neighbouring revolutionary states would rally to kick them back out, but Luzon doesn't have any neighbours.
If earlier events don't butterfly-away the creation of the Brookes' Sarawak anyway -- for example, by James Brooke going to work directly for the new Philippines government in some way -- then this alliance seems unlikely to me. Even if his Sarawak wasn't at least nominally a vassal of Brunei in the early days, which I think was the case, he'd have favoured (and almost certainly have received) an alliance with Britain instead.1841 - Kingdom of Sarawak is established by James Brooke. At the same time formed an alliance with the Philippines. James Brooke later married a daughter of Andres Novales.
Or maybe the fact that he's from the Army makes the Navy less likely to support him?Since Novales is from the Spanish Army, the Spanish East Indies Navy may also rebel.
If so, what will be his style of leadership?TTL, Yes this is an issue which Novales has to resolve. Difficult this may be, these are the same issues that Spanish America also had to resolve in OTL after independence from Spain.
What if Novales decided he's contended with the northern part of Borneo (Sarawak, Sabah and even Brunei)?If it does modernize within the time frame, it will start absorbing the moro sultanates, buy up north borneo, merge with White rajahs in sarawak and become the protector of Brunei which are still independent from Europe by this time. After that, freeing up the nations under the Dutch Colonies will probably be the top priority and absorbing what is modern day indonesia.
What if Novales decided he's contended with the northern part of Borneo (Sarawak, Sabah and even Brunei)?
If so, what will be his style of leadership?
Well, if he succeeded in liberating the Philippines, he's still caught in the conflict between liberals and conservatives (and their friar allies) in the government.In my opinion, he was a very capable military man and experienced Napoleonic wars first hand. He was very passionate. He was envied by all peninsulares soldiers probably due to his success both locally and the Napoleonic wars. However, he was politically naive like how Bonifacio was in OTL. His story however is good and very dramatic way to start a POD or an alternate timeline for the Philippines.
How about Spanish as lingua franca?The Philippines will definitely need to come up with ways to maintain its territories since the issue of ethnicity and language might go unresolved in this scenario.