There was a post recently http://www.etiquettehell.com/smf/index.php?topic=126586.0
talking about Magic Words. It was very well written and powerful. Basically it stated that there are no magic words that will make your mother change her behavior with out causing fall out. You have two choices: either keep silent (ie things stay the same) or speak up and deal with the fall out. There are no words that will make your mom understand your position without causing some kind of drama. For now, things are ok and you are able to make plans without her noticing you aren't eating at her house but there is nothing you can say, other than the truth, that will magically make her not blow up/cause a scene/have hurt feelings when she notices. You are not responsible for her drama, or her feelings. You are not asking for anything that is SS.
TBH, the milk and cheese thing I don't really get but that's because my grandparents owned a cattle ranch and dairy farm and the expiration date on milk is a suggestion, to a point (3 weeks is a bit over the top). If it smells bad then don't drink it was the rule (not that milk lasts more than 4 days in our house). The meat thing I absolutely understand having had some very questionable meat served to me as a missionary kid.
It all comes down to this: at some point you are going to have to hurt her feelings to keep your kid safe. Heck, at some point you're going to have to hurt your kid's feelings to keep them safe. It will be ok, the world will not end and even though it will change your relationship
with your mom, it will be worth it. You were probably brought up that Mom's feelings were all important and your's didn't matter. Well, you're not a kid anymore and you get to make your own choices...one of those is, you, and your family, don't eat at your mother's house. I promise it will only hurt for a little bit and be a lot less stressful in the end if you gently tell you mom the truth. Doing so is the hard part and I'm still learning that with my Mom. Good Luck (hands you shiny spine polish)!