Immigration OR Tambo exit and SAA

Yesterday I got to OR Tambo international Airport early. My flight was at 19:05. I checked in my bag around 5:20pm. Got to the the security check where there was a huge queue. Finally managed to reach immigration. Yes they conduct random passport checks. This was the first time it was done to me. The officer checked my passport 3 times and he was going page by page. Scanned it 3 times and finally stamped exit. That delay costed me missing my flight. I got to gate A02 and I found no member from SAA. The gate was closed and I could see people getting onto the aircraft. I stood at the gate for some time not knowing what to do then I finally called SAA support to find out about my bag. Unfortunately the call centre agent didn't know that my bag was offloaded she said I'd find it at my destination. Finally I found airport staff who then showed me an SAA member of staff who walked me using the back route and I exited international depatures. I get to the SAA office and they are like you must rebook. They said the charge would be R500 as I missed my flight. Strange that I missed it because it seemed already like an overbooked flight. 3 months ago when I booked I selected 9A as my seat and yesterday an email came from SAA saying here is your boarding pass. And my seat was reassigned to 15F and when I saw the seating plan the plane was fully booked.

Finally I managed to walk to international arrivals and have my passport stamped and that's how the immigration officer gave me 7 days. He also directed me to the baggage claims but yet again the SAA desk was closed. I talked with airport staff that were handling bags and they said you can look over there and it seemed like those bags were offloaded way before 18:20 which was boarding time.