The good thing about the deterministic schools of a history is that they always know an outcome and just have to fit the facts into the final framework which is not such a difficult thing either as long as you feel free with the terms like "historically inevitable": it was "historically inevitable" for Poland and Russia to became the united states so to Hell with the probabilities.
But, if anything, the whole Polish-Lithuanian union even in its initial form does not look like a high probability thing and creation of the PLC probably even less so: would it happen without the events leading to the Livonian War, "specifics" of personality of Ivan IV, etc.? Would the union survive on its early stages if, say, Vitold had surviving male children and won at Worskla? Would it survive if at the time of Vitold's death his OTL grandson, the Great Prince of Moscow, was a grownup man and a little bit more talented than in OTL (say, had abilities and ambitions of Vitold's great-grandson, Ivan the Great)? There were still plenty of Orthodox followers in the Grand Duchy at that time. Could Vitold's successor,
Švitrigaila (more talented version of OTL) continue his work and make Lithuania independent? Could Lithuania at that stage broke into the Catholic and Orthodox states gravitating to Poland and Moscow, correspondingly? The list of possibilities is if not endless than at least a long one.
Well, what was a probability for an orphaned son of an obscure Mongolian chieftain to become a founder of the Mongolian Empire? What was a probability for one of the least significant princedoms of the Central Russia to became a center of a great empire?
What was a probability for an obscure artillery lieutenant to became a head of the most powerful European state? What was a probability for a son of the provincial judicial clerk from Bearn to became a founder of the Swedish royal dynasty?