Hi, When you booked and used the FTV, would that have covered all the taxes and fees or you need to use a cc to "top t up"? If the later then you need to call it back as BA won't able to charge the cc now. I think they have like 7 days after that the system won't let it.
If there is no cc involved you can still try and call back to ask where does it sit, etc.
In regards of seating you are right not much choice left on that flight. if you travel on your own there will be higher chance that last minute cancellations happens so seats could open up even after online checkin opens, so don't get your boarding pass. (use the web browser as the mobile app has the tendency to download the boarding pass automatically)