1. Book the ticket as two one-ways, not a return. It will allow you to confirm the different legs individually.
2. Waitlist any flights you may want to fly on before booking available flights (As you will need to the miles in your account to waitlist on the other flights in the first place).
3. If F is only available to waitlist at Saver level in one direction, consider doing that first and then lock in seats in J at Saver level or F at Advantage level - provided they are available, of course. Should your waitlist clear, you can then cancel the earlier confirmed reservation against a fee and have the F Saver ticket issued.
4. Act fast when you see availability; these seats usually don't hang about.