The part wouldn't happen because he would have killed Diavolo first encounter.

I disagree. He would've lost his legs, yes. But, somewhere along the run, he would've met Kochi (or say, heard him muttering Jotaro's name), asked shim about Jotaro to which he would come to learn of Polnareff's situation and boom Josuke gets involved. (Considering JoJo logic, I 100% believe this to be possible)

Do you Mean Josuke? who does Jostle mean?