1. She was confused at his error.
1-2. She was confused by his error.
2. He was confused about something.
3. He is confused in mind.
(Are they all grammatical? Which prepositions are commonly used?)
4. That is very confusing to the public.
5. That is very confusing for the public.
(Do we have to use 'to' or 'for'? Or are both okay?)
1-2 and 2 are correctly written.
3 would be correct if you remove "in mind."
4 and 5 are both fine. Both prepositions can be used there.