To address the 'how much time should I allow' part - not that much time at all

Firstly, transferring in HKG doesn't take that much time. The minimum connection time (on this itinerary) is 50mins and is easily doable. If it even get close to that/below that due to delays of the inbound flight, there will be CX staff meeting your flight at the gate upon arrival to help you out. This can either be:
- Escorting you to the gate
- Rebooking you onto a new flight

I would just book the minimum connection as I think Osaka has more to offer than HKG airport

The Premium Economy product is amazing, especially on the JFK-HKG route given the length of the flight (amongst other factors). Elevated service and smaller/quieter/quainter cabin of around 30-40.