I would pay revenue for the inbound now, as a single, if the fare was low. If that gets cancelled you may get a surprising amount back.

I would then separately book RFS Avios redemption for your best current guess on the outbound. This gives the best flexibility until 24 hours before departure. If you really wanted, you could also book the other flight on RFS, and be sure to cancel in time, that means the £35 per person redeposit fee is the cost of this flexibility, furthermore you would be bound to pay that. Personally I wouldn't book that second RFS, I would keep an eye on the loadings and make a judgement call as to whether last minute RFS will come up, I don't think I've ever come unstuck as a result.

If you decide to book the inbound as RFS, it would be important, given your context, to make it a separate booking.