The first time was in March-April 2012, when I was still in Canada visiting family and relatives, and Sandra had returned to Australia. I returned to Australia 3 weeks after she.
It sounds strange, but is it correct?
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
... Sandra had returned to Australia. I returned to Australia 3 weeks after she.
This is not correct.
While it may be true that what you mean is "I returned to Australia three weeks after she returned to Australia", in this case syntactical form trumps meaning. Formally, bare SHE is the entire object of the preposition; it is not apprehended as the subject of an ellipted clause; it must therefore be realized in its objective-case form, her.
Nominative-case she would be employed after a preposition only when it is followed by a verb of which it is the subject—more precisely, when the object of the preposition is a clause of which she is the subject.
Sandra had returned to Australia. I returned to Australia 3 weeks after she did.
This is true in all registers. The most rigorous pedant (e.g. me—or I) would not write after she here.