Just a further thought on this - the BA Premium Plus Amex vouchers open up additional availability over the traditional reward seats that may be booked using Avios. (The criteria for opening up additional seats this is, unfortunately, somewhat opaque and I don't think fully known. I'll spare the details at this point)

Given that you are seeing 7 seats on a given day, I strongly suspect you are seeing the extra availability which is afforded by use of the voucher, as I can only see 2 outbound dates with even 5 reward seats (1 in Jan, 1 in March)

This maybe doesn't alter your strategies, you would still be looking at (minimum) 2 bookings to use both vouchers, but I must confess I don't know if the telephone agents also see the additional availability afforded by the vouchers, maybe someone else knows.

If the agents can only see the reward seats, and not the additional ones afforded by the voucher, you have something of a quandary as you will be torn between:

a) Doing this on the phone, where the agent can immediately secure the seats before processing all the details but might only be able to offer 4 (between you)
b) Doing it online (at T-355) where you might see additional availability, but there's more than a decent chance that by the time you enter the details and go to pay for them, the seats will no longer be available as they will have been secured by people on the phone...

I'll let others chip in at this point, I think my own instinct would be to go the phone route and try to secure 5, but then work out a way to add a booking for the 5th later if you only get 4.