How come a mechanic wasn't able to reset the brake sensor but I succeeded?
 

I have a 2009 Mini Cooper.

My brake and low tire pressure lights came on. When I replaced the brakes (I brought my own parts) the mechanic showed me that when he tries to reset the brake light using OBD2 it the error code comes back. He said he thought the new rear wear sensor I brought was faulty. The front one works fine. He said he sees several error codes related to electrical problems, which he believed was why the brake light failed to reset. He suggested to come other day and he'll take a look. If it's something easy, he won't even take money from me.
After I left the shop I looked up on YouTube how to reset a brake light without OBD2 and was able to reset it easily (Service Information -> Brake -> Reset). As a result, I no longer have brake lights or low tire pressure!

1. Was the mechanic incompetent or could there actually be a case where you can't reset using OBD2 but can do it from the car menu ?
2. If the brake wear sensor was bad, I wouldn't be able to reset it, right? If it brake light turned off after the reset, that means the sensor is not faulty, right? Or does it mean nothing and it takes a while for the computer to determine if the sensor is bad after a reset and it might turn back on after a few miles?

2. Correct; if the new sensor was faulty, the fault would not clear.

Remember, the brake light will come on for many reasons ... faulty brake pad sensor, worn brake pads, brake fluid level is low, other fault in the DSC system, fault in DSC module etc.... Most faults require troubleshooting by a competent mechanic. What I see (even with dealer mechanics) is that most people throw parts at the fault.

As for your mechanic's level of competence; nobody on the forum is going to know the answer to that question.

Regarding the mechanic's level of competence I was really wondering maybe he was right and there are cases when you can't reset fault using OBD2 but can do it from the car menu? I don't think it's true, but worth asking here

I work on a lot of different makes of automobiles in my shop, and I've never experience a situation as you have described. Perhaps your mechanic's diagnostic tool was defective, had a bad connection to the OBDII port (DLC) or needed a software update for the vehicle he was working on.