1) yes, you can put to verbs together. In your second example (我想有工作) 工作 would actually be your second verb, if I'm not mistaken. (so it would read: 我想工作)
2) In order to negate a verb 不 goes before the verb. In some cases it seems like it goes after, like 是不是, but when you break it down it's really 是 不是.
3) a) 牛奶是一个产品
b) is fine
c) 纯牛奶有高的脂肪含量
d) 这是什么? is the more formal way of saying (and writing) it I think. I've really only seen 什么这个 in spoken form, but I could be wrong.
A native speaker may be able to give you the how's and why's for these.