CEL: P2181 Audi A4 2.0T convertible
 

Hi,
2008 Audi A4 2.0T convertible Car has been to body shop to repair the bumper as it pulled off during parking incident. After I got car repaired this Check Engine Light came. P2181: Coolant System Performance.
I don't know much about car and tiered of Audi car repair cost, so please help.
These are my observations:

1. Car coolant gauge gradually reaches mid way and stops there, no issue.
2. But I smell engine(something) burning smell after I drive for 30mins and also I can feel engine hotness near hood as my be overheated?
3. At start some pleasant oil burning smell comes.
4. Car has oil leak for long, so many times mechanics repaired but never fixed!
5. Oil changes comes very often(like 3 months, less than mentioned time and miles) and did oil change every time.
6. I think car is drinking too much gas! With full tank Premium gas (91) I get to run only 150-170 miles on highways! (is it normal?)
7. By any chance, body shop repair people done something wrong?

Please advice me what are the things need to be done to fix car completely so that it will run in good condition. It will be helpful if you can explain me every point I mentioned and give me more information on each.