As to / As for

English as a Second Language English Vocabulary, Grammar and Idioms
1
  1. Why she came here in the first place, I’ve no idea.
    1a. As to why she came here in the first place, I’ve no idea.
  2. As to Sunday’s meeting, I won’t be able to attend.
    2a. For Sunday’s meeting, I won’t be able to attend.
  3. What we did last night, that’s a secret.
    3a. As for what we did last night, that’s a secret.
    3b. About what we did last night, that’s a secret.
    3c. Regarding what we did last night, that’s a secret.
    Please correct all and comment.
    Thanks.
2

2a is incorrect.

The others are all possibilities to one extent or another, but as they are almost all follow on sentences (they relate to something said previously) it is impossible to say whether they would be correct within particular contexts unless you were to include the preceding part.