Sorry to hear that - i believe you acted in fair manner by not letting the entire plane waiting for you.
On the other hand I understand you anger, especially because you did receive the boarding passes for your seats as opposed have the seats assigned months before in "manage your booking" or even "online check-in" which does not give you a guarantee that you will get those seats.
I am surprised that EK's ground staff came on board to solve this. Normally, it is the purser who has the authority to deal with these kinds of issues. He/ she would also have assigned you seats in F, not to speak that he/ she would have taken the right measures to deal with PAX who do not obey to FA's orders.
Was the purse (brown uniform) with you? He / she normally deals in F and J (First and Business) and there is a supervisor for Y (eco)..