Off road

I have a 1993 Toyota LandCruiser 4.2 GX and have been driving it for years without problems. I have taken it for regular services. The last service was done about 3000km ago.

About two weeks ago I had the radiator flushed, as I was planning a long distance trip. After the radiator was flushed the engine started overheating. It never went into the red, but I questioned the mechanical workshop and they then told me to bring my vehicle back where they flushed the system again and then took the thermostat's inside out.

While flushing it, the pipes weren't disconnected from the engine. The vehicle still overheated. According to the mechanical workshop the head might have cracked, but they cannot give me any clear indication of what might have happened. 
 

My questions are:

1. Is it normal to have an engine issue after flushing the radiator?
2. Is it normal to have this issue if I have never pushed my vehicle into the red and only drive it at 80km/h and have NEVER abused my vehicle.
3. When I took my vehicle back to the mechanic's workshop, it did not even have a miss in it, but the mechanic was pushing it to the limit for about five minutes while I was standing there. Can that cause damage to the engine?
4. Could the dirt in the radiator go into the engine when they flushed it, as they used powerful equipment to do that?
5. Could this dirt have caused the damage to the engine?

I am really very upset about my vehicle.