'1 in 5' is different from the others.
If five people share a cake we don't say they get one in five each.
Great! Rover, you could notice that. I really thought that they were interchangeable by Emsr2d2's first reply. Anyway, now I know that they are not interchangeable but mean the same because of [or thanks to] Emsr2d2's second response. I, however, think that "a fifth" and "one fifth" are interchangeable, while "one in five" and "1/5" are interchangeable too. Am I actually right?
I would not hyphenate "one fifth" unless it were an atributive adjective.
For your #3, I wouldn't say never. In a liquor store, a customer might say "I'll have two fifths of Johnnie Walker Black, one fifth of Jack Daniel's, and one liter of Absolut.