get Isabela and Gaston(The first somewhat politically green and the second very gung ho)
Well he is a complicated figure. Twice during the course of the conflict, Gastão sent a request by letter to Pedro II to authorize him to fight in Paraguay, but on both occasions, to his great disappointment, He was denied. He agreed with the duke de caxias on the issue of paraguay (When he arrived in Paraguay, he reorganized the Brazilian army and dismissed officers accused of looting enemy territory). The Viscount of Taunay said that the count revealed "great strategic skill, the patience of an experienced captain, indisputable courage and cold blood". He also tried to get involved in government like this with his wife (but Dom Pedro wouldn't let him).The government will be a mix of congress and empress, with the empress being weaker in the beginning. He will be excluded completely from the government. Interestingly, most of his fame is invented by the Brazilian elite (like Rui Barbosa who didn't like a foreigner to have so much influence in the country).
or even a particularly stubborn Brazilian diplomat who demands the "fair share" of Paraguay in the talks with Argentina we could see something like a tense border between the two countries as they both take Paraguay apart and divide amongst themselves,
I think this is likely to happen if the country is shared because Argentina will want most of the country for historical reasons, but Brazil will have the biggest army. Brazil and Argentina hated each other during this period. Not only that, but depending on the partition taking place, Brazil will have more access to the Plata Basin, which was a nightmare for the Argentines. The most likely is basically a continuation of the war in Paraguay, but now between Brazil and Argentina.
if things heat up again, we could see a Second Paraguayan War that would without a doubt be won by Brazil thanks to having a large experienced army and navy as well as Argentina's constant troubles between it's federalist and centralist factions, either way Brazil gains even more territory from Argentina and become the undisputed master of South America.
More likely, Brazil will create a puppet state in the Entre Rios region (maybe a protectorate) and annex Paraguay as a whole. And wanting to push the country's debts to Argentina (either that or demanding reparations to pay the debt). So, probably after this war with Argentina, Brazil will have another 2 or 3 wars until everything settles down. In addition, Chile can attack Argentina, basically the South American cone will have many more wars (there is also Bolivia and maybe Peru in this matter)
with Isabella in power and with the support of the military and these veterans and other sympathizers she could end slavery earlier and one that has a few caveats to help out the freedmen as well as giving compensation to former slave owners, there would be a push back sure but with the support of the army as well as the power of the state, she could crush any opposition.
Depending on the situation, this group may encourage a war with Argentina as a way to inflate the number of former slaves in the army, creating a way for slaves to achieve their freedom more quickly. Regarding the monarchy, it probably survives but it will be different, I don't know if the country will be a democracy like England (maybe the emperor will keep more power). Anyway, I think it's likely that the throne will be passed to Pedro the Third (son of Isabel as soon as possible probably when he turns 18 in 1893). I also don't know if the dispute between Brazil and Argentina will be taken to WW1 (with one side going to the central power and the other to the entent, I think it's unlikely)