Negative Fuel Trim

Hello, I am new to the forum and trying to help my sister's mother in law fix her 2010 Aveo5 LS with 1.6 L engine. She has a p0420 code that pops up intermittently. She took it to an independent repair shop and they replaced both 02 sensors and the catalytic converter. It would pass the self tests for awhile then throw the code again. Apparently they replaced the cat 3 times as she took it back each time it threw the code again, and now they say they don't know what to do/can't fix it. She doesn't have a lot of money to spend on more repairs.
I bought a computer software based obd scan tool and pulled the 0420 code. The O2 sensors and cat appear to working normally: O2 sensor 1 (pre cat) fluctuates between .1 and .7 rapidly and 02 sensor 2 (post cat) is pretty flat around .6. It seems to run okay though not super smooth, but I'm not really familiar with how it ran before either.
The fuel trims are fairly negative though. After a reset, with long term fuel trim at 0, as the car is started, the short term starts going down fairly rapidly at idle and within a few minutes has dragged the long term fuel trim down to around -17%, where it will stabilize as the short term fuel trim fluctuates between +/- 3 or 5%. As I go up in Rpm the both long term and short term will come up a little, maybe -13% or so.
I understand this means the computer is detecting a rich condition based on 02 and MAF sensor airflow readings. I have removed the air filter temporarily as it was a bit dirty but it didn't seem to help and looked at the MAF, it looked pretty clean. I haven't pulled out the injectors to check for leaks yet. Any ideas? Thanks!

1) Check for vacuum leaks
2) Make sure the intake hose to the throttle body is in good shape
3) Check for oil around the spark plugs. The valve body covers are prone to leaks. Just pull the wire out of spark plug and make sure there is no leaks there
4) One of the fuel injectors is a bit leaky

Oh yeah, I used to live in Raleigh. Glad to see an other NC fellow on the forum. Welcome!

Did you pull the plugs and chek them? If not, do you know if the shop did?