Coming from Disney, it seems crazy and overwhelming that something wouldn't have Express Pass but, honestly, we don't do "rope drop" or early entry and I have never had a problem and, except for one time, have never had an exceedingly long wait for any of the Harry Potter rides.
If you are into early entry, and USO is the first park to open that day, here is what I would do - head to USO, do Gringott's and Olivanders and then have a little stroll through Diagon Alley until it nears opening time for IOA when you should head to the Hogwart's Express. When you get to IOA, ride FJ and whatever else you are interested in at Hogsmeade (Dragon Challenge, Flight of the Hippogriff). You can then choose to either spend the rest of your day at IOA or ride the Hogwart's Express back to USO and spend the rest of the day there.
I doubt you will have more than a 20-30 minute wait for anything.