Should I redo this wall to avoid a mold sandwich?
For a pure shower (not bathtub), I always put a membrane on over the backerboard.  Redgard is my preferred choice right now so I would be fine with that choice.

I really don't think there is anything that you need to do.  If I am putting plastic behind my backer I should see it sticking out under the backer and over the base in the picture - it would be covered by the lowest row of tile.  It's not... not the biggest deal in the world if you have a membrane plus backer.  

Final take: There is nothing wrong with Zhentar's advice.  If I were doing a bath/shower combo it is exactly what I would say.  However with the use of caulking for all of the gaps I would one a solid membrane over the area.  The fact is the caulk will dry out expand/contract and may leak after 5 years or so.  You are binding the caulk to larger materials like the DS which will have different expansion rates.  With a membrane you have one solid surface that is made not to crack.