Regarding switching to J later: Yes. I have done it. Waitlist F first then confirm J. You can cancel the J ticket and redeposit the miles for your F ticket if it becomes available later. I suggest waitlisting all the flights that would work for your schedule.

Assuming SQ does not fly any planes with F to MNL (I havn't checked) you will need to make a mixed class booking over the phone. Don't take any nonsense from the helpdesk about pricing it as two awards!

And good luck!