Air India Express - Seats and Aircraft Changed without prior intimation

My PNR BZD**C from Bengaluru to Trivandrum on **th November ****. Seats booked were **A, **B and **D. When we got into the flight, we were surprised to see that seat number **D did not exist. Only when we enquired with the crew did we know that the aircraft was changed for some reason and this seat number wasn't even there in the plane that we boarded. Moreover, the other two seats were at the emergency exit, which I had clearly avoided when I pre-booked the seats. My **-year old son was keen to take the window seat and he was not allowed because it was near the emergency exit door. But I had booked a different row (not near the emergency exit door). There was confusion and the crew members didn't know what to do. They called the ground staff who tried giving us three different middle seats stating that the flight was full and there was no other seat available. So what about the seats that I pre-booked? Well, the ground staff had the audacity to tell me that the flight was getting delayed because of me and suggested that I take an alternate flight in the evening if I wanted a window seat, which I had booked. And we woke up at * a.m. to catch this early morning flight and faced a lot of embarrassment in front of fellow passengers first thing in the morning when I was going for our Diwali holidays with my family. Later on one of the passengers couldn't see our plight and helped us by giving his seat and he moved to another seat since he was travelling alone. I thank that gentleman.
I insist that the airlines must apologies in writing and refund the amount to me for causing such inconvenience, confusion and embarrassment without informing us about these changes.