A friend with a similar problem said she started to respond EVERY time her coworker tried to talk to her by waiting a second or two, looking up, and saying (distractedly) "Oh, were you talking to me? I was concentrating on my work and didn't hear you." She said it only took a couple of days before the woman caught on that she really didn't want to chat!

As far as the food -- I think I might be tempted to burst into tears every time she offered me something sweet and say, "I'd rather not talk about it."

I was generally lucky with office mates, with one exception, and he ended up getting fired. (He was an alcoholic who started drinking again.) I don't envy anyone who has to spend their work time with a coworker who drives them nuts!