There were multiple uprisings over the course of 19th century, with the biggest revolution in 1905 (until 1907). This was a direct uprising against the reduction of autonomy and forced Russification initiative launched by Alexander III in Baltics, Poland and Finland. As well as the ban on the...
Latgale only joined the rest of Latvia, because the locals decided that they want to be together with the rest of Latvians in the Latgale council. If theres a United Baltic Duchy, they might not decide to do that. Of course it also depends on what would have happened with Russia in this...
As a Baltic German from Latvia, here are my 2 cents about this.
Since 1880s Latvians and Estonians were slowly pushing for autonomy, especially the push was increasing after the autonomy for Kurland and Livonia was reduced and the Russification program of late 19th century was introduced...