I don't think USGA would happen, FF seemed to have moved away from the proposal towards Trialism. Not that he really wanted Trialism since the Hungarians would be powerful and he didn't like Slavs at all. He just saw it as a inevitable move to solve the "Slavic problem".
If USGA happens I think Popovici's proposal would be quite altered (I really doubt Carniola dominated by Slovenes would ever be accepted by the Austrians for example).
I do not believe Trialism could work as a long term solution because the Slavs were never a united bloc in the first place. Possibly one of the few things a Slavic parliament could agree on would be further decentralization in favor of their own regions, Bohemia, Croatia, Galicia. Even if it could work for a while, what about the Romanians or the Italians? After we get a third parliament, we could see a fourth and then a fifth. By then it seems more likely that Vienna would be looking at a federal solution instead of giving everyone their own Ausgleich. Perhaps OTL India could serve as a model of how that could work?
But that could a be preempted with the right POD like a Hungarian insurrection and the Empire goes back to being a unitary state for a while. So instead of a total federal solution, AH could evolve instead into a unitary but decentralized state like modern day Spain in OTL after the Franco regime ended.
That is the two ways I could see Austria-Hungary surviving the twentieth century.