"się" is usually not used at the end of the sentence. Such word order is not natural.

If you use Formal You, you cannot omit the subject. "Dlaczego się śpieszy" without "pani" is just "Why is he/she/it in a hurry?", and the subject would need to be known from the context.

I obie formy są akceptowane.

Yes it does.

Also it's Dative of 'co', which you may have suspected.