'Hard' in this context probably means 'difficult'.

Hi Лариса, Peter is correct: "hard" more than likely means "difficult," although there is still a small chance that it means "impenetrable" depending on the context. Think of it this way: "Is is that hard?" is a shorter way of asking "Is it that hard to do?" I hope this helps answer your question. Please let me know if it doesn't. EDIT: Wow, I can't believe I didn't read the whole question. Silly me =(

You're correct -- in this context, "that" has the same meaning as "so."