I think if you had explained at the gate then things would be no problem, even for in cabin reallocation of seating for your son would be no problem. The tallinn call centre is quite powerless and simply does not have the appropriate access. There is no point trying to force a square through a circular hole.

In such a situation the important thing is to get there, I would accept a LD3 just to get there. It is better to do things in person in my opinion, harder to reject. I understand that you are in a difficult position but perhaps a more concilliatory tone would be helpful. To be harsh for a second, from the transcript it sounds like you are using your son's disability as an excuse to dictate terms - I am sure there are a range of reasons people can come up with the bypass seating rules. Tallinn call centre is simply not equipped to judge case-by-case.