If you google Singulair side effects in children, you can find websites with some REALLY serious complaints about aggressive behavior, night terrors, and more. No way I would ever put an infant on that stuff. I'm really curious too how it is supposed to "help" the immune system???
I would say that there is probably still a food in your diet that is bothering her if she's continuing to have ear infections and a runny nose and isn't sleeping well. You don't necessarily need to do a full elimination diet, you could start by taking out all the top common allergens- dairy, soy, gluten, eggs, nuts/tree nuts, fish/shellfish, corn.
Are you keeping a food journal? If you make sure to vary your diet every day (don't eat the same things over and over), you should eventually be able to spot patterns in symptoms with what you eat.
Also, since she's already had antibiotics so many times, her gut is probably low on good bacteria, and that could be contributing to her reacting to things. You might try a good dairy-free probiotic for her and see if that helps.
Mom to DD1 (10/07) and DD2 (3/11) I blog about our life with food allergies and eosinophilic disorders.