I am getting the dreaded P0172 code

I recently bought a Chevy Sonic LT 1.6 l (only produced in Mexico, where I live). The test drive was smooth, a little hesitation but a generally smooth ride. I listened pretty carefully under the hood. Did not perceive any vacuum leaks by ear or other weird noises. Hoses, belts and such looked ok. First thing I did was I changed the oil, did a coolant flush, changed the timing belt/tensors and the three "retenes" (I do not know how they are called in English), which were leaking. I guess the guy who sold the car to me reset the ECU because after a few kms, the CEL came on. A scan threw P0300, P0173, C0561. The last code is for the ABS so I figured that one would disappear if the others were dealt with. I checked the spark plugs, and they were clean, not white, nor black which I thought they might be because of the P0173 (I am not a mechanic but I know some basic stuff). I changed the plugs anyway. cleaned the MAF and took the car to a mechanic to clean the injectors, which he did with a pressurized can. He reset the ECU and I drove the car through its cycles, and it ran smooth, better than before. But I still got a P0172. A real-time scan by the same mechanic showed the only anomalous reading LTFT = -29 and fluctuating. Oxygen sensors fine, and other parameters that I am not familiar with he told me were ok. He suggested I replace the MAF, which I did. It is a BOSCH.

OK, here are my questions. Since the mechanic already reset the ECU after the scan, and since I drove the car only about 10kms to get the MAF, I figured I did not need to disconnect the battery or anything because the ECU was just reset so no need. Just popped the MAF in. I let the car idle for ten minutes then drove it ~25kms. Very smooth ride, engine idles at around 800 and no hesitation. But should I have disconnected the battery to reset the ECU again?

Second question: the car ran fine from the first test drive. If it wasn't for the CEL going on, I would never have noticed a problem. Why did I get those codes without any synptoms? I know the CEL goes on if the Trim goes out of the -25 to 25 range. So I am assuming the issue was not severe.