I don't like the flow of it. To me it sounds like a command "
Rain,
Hannah"; like you you are telling
Hannah to
Raine.
Hannah, I like but agree with you about using it as a middle name as opposed to adding to the multitude of newborns with the first name
Hannah.
Raine isn't really my style, though I do like
Reina.
To my ears short names tend to sound better as middle names so I think of any good middle names for
Raine, except maybe
Elizabeth.