You should absolutely check out the TSB provided by CSA above. If that doesn’t help then get a new mechanic. (Or maybe you should get a new mechanic anyway).
If you have a P0125 and then some A/F sensor (O2 sensor) codes you don’t touch the A/F sensor issue until you’ve cleared up the P0125.
The P0125 is about the engine not getting hot enough to get to closed loop operation - closed loop operation is where the computer is using the O2 (A/F) sensors to do the fuel/air mix. So if there is an issue such as the P0125 then you don’t trust anything from the O2 sensors until that is right.
Do you know why the coolant temp sensor was replaced? Was you thermostat checked? Was the actual coolant temp measured? Was the temp sensor actually checked?
Anyway, hopefully cleaning up the connections mentioned by the TSB is all you’ll need to do. But if not then worry about your choice of mechanic.