I sympathise with your situation. Whilst I cannot help with the ticket situation. I have taken my son on 5 long haul flights, 4 of which when he was under 2 and the most recent, last week, when he turned 2. As I tend to travel with him on my own I did appreciate the fact he had his own seat this time. However, I did travel with him when he was 22 months old and he is also tall for his age and it was ok, so if you are unable to make any changes I wanted to try and reassure you.

Although his legs hung over the Britax seat on the bassinet, he did sleep fine in it and having the extra room around the bulkhead meant he could stand and easy to do colouring/other activities.

As you are economy, it might be cheaper to try to upgrade both your seats to WTP rather than pay the full fare for a new ticket. The extra room does make a big difference.