Aside from what schleppel said, Mitarbeiter emphasizes working together (literally a co-worker), while Angestellter emphasizes the superior-inferior relationship. Btw, a blue collar worker would be "ein Arbeiter"

1. It's not a common phrase in standard German - maybe dialect. But people would understand you. Better would be "Ich mache Suppe zum Abendessen."

2. "Angestellte" do white collar jobs, while "Mitarbeiter" are employees in general.

3. "Sofern" ist just a very formal synonym for "if".
   "Sofern ich das Geld habe, kaufe ich es." - "If I have the money, I'll buy it."

"Auf dass" isn't used very often anymore. I can only think of toasts or a speech at a gathering. "Auf dass es uns gelingen wird!" - "To our success" (not a literal translation - more like " We drink and hope we will be successful". But that doesn't sound very "speech"-like)