Waitlisting under this fare is permitted (except for the first international sector), if CX permit waitlisting in the D bucket.

Ask the call centre to get you a waitlisted reservation in D class.

I'm not sure they'll allow you to hold space on two separate flights, one on a waitlist and one confirmed.

If the waitlist clears, the status will change from HL (held on waitlist) to KL (confirming from waitlist) and the agent will get a message in the queue to deal with it. The other sector in the lower class should be automatically cancelled if it's on the same plane, otherwise I think it would have to be done manually. The agent will need to acknowledge the change and revalidate/reissue the ticket in the new class.