If it were me and my choices were to fly Y or to pay 57.5k for a PE award, I'd spend the 57.5k on the PE. It's functionally the same as a domestic first-class seat, which isn't great for long haul but beats the socks off regular Y or MCE. AA's PE is among the best out there and everyone I've recommended it to has been impressed. Then, set ExpertFlyer alerts (it's worth the $$) and if one goes off, upgrade to J (no cost for that).

As for odds of J (U) becoming available, it's probably pretty close to zero until you're within a few weeks of departure. For example, there's true saver availability on AA in business on April 8 and 9.

Also, getting LAX-MIA in J on the 77W is a nice consolation prize for being suckered into paying J miles for PE on MIA-SCL.