I’ve done research into this, so just a few thoughts:

- Never heard of the Black Hand being against Trialism to the point of plotting to kill the pro-Federation Archduke over it.
- Highly unlikely that Austria would have ever given up Marburg (Maribor), let alone Istria. Even Hungary only got one port city as an outlet. In fact I don’t remember ever seeing Marburg on a Trialism map in the Croatian Kingdom, honestly.
- This was, as I recall, the earliest and most ambitious proposal. Most Trialism proposals were eventually scaled back to centre around Croatia-Bosnia, excluding Carinthia and Carniola (Slovenia).

Regardless it’s an interesting idea but it wouldn’t have resolved the Bosnia Question the Austrians created when they stole Bosnia and started pushing out the Serbs in 1878, or the issue of the minorities in Hungary, etc. You can look into the Austrian Federation that the Archduke supported as well, if you’re interested.