There is a myth that Ester oil is compatible with 12. I would agree that a TINY AMOUNT of ester staying behind would not hurt, so maybe it is compatible in that way, but don't use Ester solely in an R12 system. Flushing with dry nitrogen will NOT remove the oil. It could be that you could flush the compressor with mineral oil without removing it, but then how will you empty the compressor so that you can measure in the right volume of oil. This is important, too much oil could lock up the compressor and too little will wear it out.
I can't tell you whether you should find another tech, but I sure don't like what I'm hearing.