Good attempts everybody.
I can't really think of a case that is not suitable to replace "どういう" with "どんな". The difference? I really don't know honestly. I think it really a personal opinion/preference. It sounds to me, どういう sounds little more formal. But as far as I know both are pretty much exactly the same.
実際=in fact, actual、事実=fact、実は=actually, in fact, to tell the truth、本当=truth, true, real、マジ=serious
Keep in mind that not all Japanese use them rightly. We can make mistakes in the usages, especially in conversational ones.
どうして、なんで、なぜ
They are again the same in the meaning. But なんで is more for a casual use. どうして and なぜ are more formal. You can reply with から for any of the three when giving the answer.
Well Elizabeth さん,
「そんな」and 「そんなに」 are actually the same. に has to be added when used before an adjective. It isn't a short version of そんなに.
But your guess is not a bad shot. Usually short versions of words and expressions are rather slangish.
I hope all of you know already that there are so many words that have the same meaning in Japanese. When you see those words and expressions, focus on how they are used in an example sentence or dialog. The differences are often the level of politeness. Yes, very often.