Is it allowed to leave my cleaning lady in the house alone with a crock pot meal cooking (she knows that I am going to work although I never told her)?
Yes, this is permitted.
The halachah concerning possible switching of meat refers to a case where the non-Jew might gain from the switch. There is surely no gain from switching a half-cooked piece of meat, and this won’t be a problem.
In addition, there is always a chance that somebody will come home from work (unless the non-Jew knows that there is no chance at all), and this will also be enough to prevent the concern.
See Yoreh De’ah 118:10.