It doesn't really help but lactose intolerance is actually not an allergy, it's a normal development your body goes through at some point in your life. Anyway, try lactofree products. I don't know where you live but they're widely available here in the UK. There are also little tablets you can take to help you digest dairy products.
Yes. My friend got really sick from diary, and now she's fine, but her baby is lactose intolerant
My sister did with my nephew and she still intolerant. My nephew is 16 now.
Absolutely. It can cause all sorts of new allergies.