LOL Marina! Ds did similar to my (also chic and childless) sister last week - walking down the street: "I'm hungry and thirsty" - 30 seconds later: "oh look there's starbucks, let's have a biscotti!" She fell for it.
Bells 2 what a horrid situation for you, it must be very stressful. I can sympathise as we had a similar(ish) situation 3 years ago with our very first nanny. She was great with ds - endlessly patient, played for hours, spent ages making up all sorts of interesting (!) purees, taking him to all sorts of different activities, got him into a good routine etc etc, but she was really horrible to me and ignored how I wanted things done (eg she gave him apple juice repeatedly - at 4months old - although I said clearly 4 times in all I wanted him to have just water, she also got aggressively cross with me for giving him milk in the middle of the night when he was genuinely hungry, she would try to make sure he was asleep when I got home from work etc etc). Overall, she saw him as her child rather than mine. In retrospect I should have heard sirens and seen flashing lights when she had admitted at interview to having a yearning need for her own child. She was so consistently rude to me that I found myself dreading going home in the evenings, which I now find absurd.
I really struggled with what to do (dh thought she was great, didn't see the problem and thought that it was all just a problem in my head arising from inner conflict at leaving my child!) - although unlike your circumstances she hadn't been with us long enough for ds to become attached, it was clear to me that she was great with him and that he would flourish with her.
Eventually I reached the conclusion that in employing a nanny my purpose was not only to look after ds's interests but also to facilitate the whole balance of life generally. She was making me unhappy and that wasn't good for me, or, I reasoned, dh or ds or our life as a family generally. Although she was our first nanny and therefore I didn't have experience of anyone else, I felt that there had to be someone out there who would work much better for us as a family. So - against dh's virulent opposition - I let her go after her probationary period. Our next nanny was a fantastic Australian girl who we all loved and who stayed with us for over 2 years, so in our circumstances it was the right thing to have done.
I know what you mean about not being petty, and no nanny out there would be 100% perfect, but this is clearly more than that. You're obviously unhappy and that can't be good for your son or the rest of your family. I've just recently remembered (having experienced it then for the first time) what an absolutely enormous difference it makes, to ALL aspects of life, getting unsatisfactory childcare arrangements sorted out. It is an upheaval to change arrangements and it takes time and energy, but when you make a bright and happy new start, you feel like a new person who could conquer the world! I know it's harder for you in that your son is very attached to her, but presumably she would move on naturally at some point anyway? And aren't you on maternity leave at the moment? (sorry if I'm getting confused and you're not) - if so might it be a good time to have a changeover, while you're around to hand over?
What doesn everyone else think? The message I got, loud and clear, from the collective wisdom when I was having my recent dilemma was to follow my instincts - having done that twice now I do absolutely think it's the right thing to do. [You're paying her to make your life better, not to depress you.]
Bon courage. Please let us know what you decide to do.