Perhaps the radiator cooling fan thermostat wiring was broken during install.
It's like cheese and has happened to me.
Or the inlet to rad is cracked, very fragile plastic.
The I/C you put in is free flow through, I put a cheap ebay one on and had no issues.
Mechanic doesn't know much or not volvo or not interested in your problem.
Sure you may need new radiator, it's really old now but diagnosing is first priority.
Have they tested coolant for gas?
If they don't know how to do it walk/drive away quickly to a proper shop.