It’s not clear whether you’re the only person who can detect this. If it is, then you having a sensitive nose might be a significant factor. But you said your original dealer arranged two valet cleanings, and the second one suggested a fluid leak, so that suggests they could smell it as well: if so, it’s not you, there is something wrong. You shouldn’t have to put up with something that’s “musty and stomach turning” and seems to make it unpleasant to use the car, that’s not what you paid for. If you’re contemplating rejection, your case would be strengthened by getting an independent report to confirm the odour exists and is sufficiently unpleasant as to make it difficult for you to use the car.