I've had reasonable success clearing HKG-JFK and vice versa as a SL (cleared 4 one-ways out of 6 attempts) and always assumed that would be an elite-heavy route where it would be hard for me to clear. For whatever that data point is worth. People spend a lot of time trying to scientifically measure the odds of clearing an upgrade. I've found it's basically down to chance depending on the particular day.

They do release more seats closer to departure. The last segment I cleared, it was actually after the ticketing deadline for my original waitlisted request and the waitlist had cleared out of the system. On a whim, I called MPC before OLCI opened and they had an upgrade seat available.

Not sure about your last point. If you mean you requested a roundtrip upgrade but only one segment has cleared so far: You can issue the ticket for the upgrade segment where you already have a confirmed upgrade seat. But this will be charged at the one-way mileage amount. You can then stay on the waitlist for the segment that hasn't cleared yet, but if that clears later you still have to pay the one-way mileage for that award, too. It's an unpleasant quirk in the system -- you can only get the "cheaper" roundtrip upgrade if both segments clear at roughly the same time.

Since they now seem to make you ticket the upgrade quite soon after you clear the waitlist, it's probably better for you to do that and be sure of getting the upgrade at least one direction. Then keep waitlisting for the other segment and hope for the best. Assuming you have enough miles that you could afford to do it as two one-ways instead of the reduced rountrip amount.