by (on/at/in or before)

[beachboy]

1)She'll be back by Sunday
2)She'll return by Sunday
3)By Sunday she'll have returned
How similar are sentences 2 and 3? Can they ever convey the same meaning?

[Tdol]

Yes- it's just a matter of emphasis- you might use number 3 for someone you're missing greatly.

[beachboy]

Following this train of thought, would it also be acceptable to say By Sunday she will have been back or By Sunday she will have been here?

[bhaisahab]

Following this train of thought, would it also be acceptable to say By Sunday she will have been back or By Sunday she will have been here?

No. "By Sunday she will have come back."