Unsorted ideas: Beginnings of Palissandria
The inspiration for that one was the Thornton expedition in 1608. In the 1800s the Tuscans basically try again at around the same time the French do. The French attempted to establish a penal colony just like in OTL, but were destroyed by the natives (which in TTL, were allied to the Tuscans)
The first French effort to colonize Guiana, in 1763, failed utterly, as settlers were subject to high mortality given the numerous tropical diseases and harsh climate: all but 2,000 of the initial 12,000 settlers died.
BUT ITTL with a different set of events occurring in France & greater Italian (private & national) interest in the Americas perhaps you see a series of private expeditions sent by trade companies to gather rosewood & other materials from 1763 - 1790s (with the permission of the French who see no better use for the colony at the time?) these private expeditions would be mostly from Tuscan & Genoan companies and in this period build up a relationship with the natives in the area selling tobacco & other high value goods in exchange for natural resources & other botanicals + working on small plantations in the region.
Palissandria in the 19th century
hmm okay so we have this 'colony' that appears at the beginning of the 19th century in the aftermath of France's defeat in the Augustine wars.. so throughout the 19th century they probably would have built a lot of close economic ties with European nations & other American colonies (buying a couple virgin islands even, which is something I forgot they had lol). In the latter half of the 19th century the 'Tuscan West Indies company' would effectively be independent from the Tuscany proper (though still de-jure under Tuscan sovereignty).
Tuscany - Palissandria Civil War
for #1 perhaps what you see in my 1860's most nations in Europe have abolished slavery and British liberals are keen in stopping the slave trade completely but Tuscany has very little control over the affairs of the colony right? so the British might try unsuccessfully for years to pressure the Tuscans to abolish slavery in Palissandria which the Tuscan government responds positively to but can't really anything about leading to an event (perhaps a conflict of some sort) with the Tuscan West Indies company.
maybe the Tuscan-West-Indies colony charter is revoked & a civil war occurs which Tuscany wins with the help of the British? but in the aftermath of Tuscan retaking the colony they realize that they can't really exercise effective control of the territory (the TWIC had all the political, economic & social contacts in the region), their military & navy are too small and there's internal pressure within Tuscany (maybe a republican or communard movement happening during the communard war arc) and so they effectively set up a home-rule system in Palissandria where the country isn't independent but manages its own affairs for the most part (but this time no slavery) with a Tuscan appointed governor