Flooring – Laminate flooring: Do I need to leave a space between the floor and the wall

Concrete wicks water, so even if the membrane under your screed layer is undamaged, if any moisture gets into that layer it's going to be redistributed as evenly as possible throughout that layer.

What you don't want to do, with a DPM under the screed, is create a moisture trap - which a second DPM would do.

Instead of a membrane, use a foam underlayment material which will keep the wood from touching the screed directly (avoiding any moisture transfer) but which will also allow the screed to breathe and thus dry over time.

Your vapor barrier needs to extend beyond the edges of the floor to prevent moisture from getting to the floor. Install in accordance to manufacturer's recommendations.

The expansion gap needed also depends on the manufacturer's recommendations - different materials expand at different degrees. The point of the gap is to give it room so it doesn't end up buckling. If you can't find a solid recommendation then use 1/2" for the maximum protection - and if the trim you install doesn't cover the gap, use quarter round.