I am delighted to hear that you have acquired a good cook, devoted to producing fine feline food. I have the misfortune to employ only one household servant, who is incapable of cooking for me. She insisted that I eat take-away food out of envelopes or dried pellets from a large bag. Meanwhile she cooks herself delicious dinners of chicken, fish, and even sometimes beef.
Theft among household staff is always a worry but there is little that we can do about it. When you adopt a human, you have to put up with their funny ways. So I think your attitude is sensible.
Catnip in your food? A small amount should do no harm. Unlike humans who use and abuse their drugs of choice, such as alcohol and weed, we are always moderate in our appetites. When I sniff catnip, it is true that for a moment or two I may behave in a relaxed roll-about fashion: but humans become drunk or stoned for hours at a time. Catnip will make no difference to your human: it's not strong enough.
I think you have a real prize. If she steals a little, so what.... just enjoy the cookies that are left.