Actually, the Paraguayan War delayed the abolitionist plans of Emperor Pedro II.
I really believe it's impossible to prevent the Paraguayan War. It's just like World War I: it could have been delayed, but not prevented. It was doomed to occur sooner or later.
the paraguayan war is nothing like WW1. the P war is dependent entirely on one man: F Lopez. Even assuming FL continues in power, war is not inevitable. He liked his ego stroked. stroke it, and he's less likely to be your enemy. Don't meddle in Uruguayan politics, and it doesn't happen OTL. If the Uruguayan Crisis passes without a war, there's no guarantee that another cause will pop up. Unlike WW1, where something was almost guaranteed to pop up. Uruguay was a little mess of a country where Argentina and Brazil liked to meddle. Paraguay was a little self sufficient country, but spending beyond it's means. FL was an egotistical expansionist who nobody took seriously. Things came together exactly as needed to spark a war. Nothing is inevitable, but the P war is less inevitable than usual.
Everything I've ever read about Brazil indicates that the war amped up societal pressure to end slavery. Pedro's personal plans may have been delayed, but society's willingness to entertain abolition was enhanced by the war.
Was Thornton that influential? Both the Argentine and Brazilian governments had interests in supporting the Reds in Uruguay, and forging an alliance of convenience. When Solano Lopez decided to military intervene in Paraguay, likely hoping for getting Urquiza's tacit support, Paraguay was put into a collision course with the Argentine-Brazilian alliance.An ATL with no Paraguay war its possible, you have at least 2 POD:
1- Get rid off Edward Thornton, the UK ambassador in Argentina and main pro-war spokeman. Before the Paraguay war, Argentina and Brazil destroyed the Uruguay army (whites against reds) and led the country more into the UK sphere of influence.
2- A huge rebellion in the inner Argentina (mostly of the arg soldiers were chained to the battlefield) led by Lopez Jordan or other, better organized, would put an end to the war plans.
Until then, Paraguay was the most advanced country in Southamerica - rails, big foundries, many factories - so probably it could keep investing in this.
Was Thornton that influential? Both the Argentine and Brazilian governments had interests in supporting the Reds in Uruguay, and forging an alliance of convenience. When Solano Lopez decided to military intervene in Paraguay, likely hoping for getting Urquiza's tacit support, Paraguay was put into a collision course with the Argentine-Brazilian alliance.
Joining Argentina against Brazil or joining Brazil against Argentina (most likely the first, as Brazil was shockingly deficient militarily in 1865) is the only way Paraguay could win, but alliance was not the mindset of Lopez.
It probably would mean that there would be some other war. Imperialism caused the Paraguayan war and is a constant in the formula that can't be removed.