It's not a matter of the average pax that relies on the airline, we're talking about a severe medical condition.

mkjr I don't know if you ever travelled with someone in such condition but it's painful and the very first thing I ever thought of doing was making a call to the airline to make sure that everything would have been ok.

Everything in such situation is worse, it doesn't just feel worse, it actually is, but to avoid that, a phone call is the bare minimum...

The OP is QFF Plat, bmi Diamond Club Silver and Velocity Silver, I think he knows that seats are available to be chosen before check in. It's not a secret for seasoned traveller.
If the computer doesn't let me select my seat and I know how important it is I would have make a phone call. Of course in certain situation the most obvious thing to do is the one we forget because we have bigger problems to take care of.

That's why the forums like this exist, to share experiences. I think that while sharing is experience the OP blamed it a little bit too much on the airline, but we can definitely feel the pain of what happened. I don't think that anybody would deny that.

We all are sympathetic with the OP, and now that is a member of flyer talk I suggest him to ask before any other trip because there are many people here that with their expertise can help him to make any future trip better.

PS By the way I always called BAEC to reserve in advance seats on CX award and they always did it.