I don't understand why you are doing 1, 2, then 3? Why not book the flights to start with using the companion voucher?

You can book one way with the companion voucher and then add the return flights at a later date if they become available.

The BA agent is correct, when cancelled there is no guarantee the reward seats will become available again.

EDIT: In addition when you cancel under 1 & 2 you will pay £35 per person per booking so at least £140 in cancellation fees and possibly an offline fee of £15 as well if you don't do it on ba.com - so potentially £170.