Dear C.,
I was in your position at one time. I was a new young mother and I wanted my son to be taken care of my way. That is why God gave him to me. However my in-laws thought feeding him adult food wouldn't hurt him. I used to get so mad. I would storm out and throw a fit!! They said it wouldnt hurt him, but in fact it did. A babies digestive system doesnt work like ours just yet, and it makes it harder for them to get rid of things like we can. So your looking at upset stomach, overweight, and maybe as you said food allergy. I have a total of two children now and even my second child was fed MY way. My baby, My way.
Their little bodies cant handle stuff like that, although some people seem to think so.
I am afraid that there isnt a whole lot that can be done with your mother, except for maybe you and your husband to split your work schedule. One days, and other nights. That way you can raise your baby properly. Plus save up and get the heck out of there. You will never be at total peace until you have your own place. Your baby feels your stress. Never under estimate your little one. Babies feel more that what you think.
Hope this helped. I will say a prayer for you and your family.