whatever ya do buddy just don't make two wonderful membranes that are parallel to each other all over because this makes a moisture sandwich which always builds up and rots... anything else is not as importnat as this, ya got it now?

What was the purpose of the green board?

The water barrier behind tile ends at the top of the tub.

I is a not a vapor barrier, but an actual membrane to stop water.

Water goes through the grout in a tile wall, and then through the cement board just fine.

How does water go horizontal through grout and than through thinset, and than through cementboard? I have a open framed room behind our shower, and never see water running down the clear plastic sheet. Is that a myth? Is that form of construction no good anymore? And how does the water get out of the bottom being it is caulked? Wouldn't this water remain captured behind the plastic?

water = humidity = moisture = presence of H2O =
not necessarily "running" as a liquid down the clear plastic sheet.

Water can evaporate behind caulking. Water can seep anywhere and pile up anywhere if it is encase between two membranes.