You want to install membrane inches above finished floor/threshold, pour floor... THEN install the hardibacker (1/2" to 1" above poured floor).
Make it so membrane ends up behind hardibacker but you don't install screws so penetrate the membrane BELOW THE THRESHOLD
... that make sense.. Here, if fold and seal all corners right and don't penetrate membrane below threshold with screws you have effectively created a sealed chamber that directs water and weep water to drain... period!
Which reminds me... use hardibacker screws (1-1/4") and construction adhesive on studs... AND when you see the drain for shower pan membrane there will be WEEP HOLES
at drain. You must add small rocks/pebbles AROUND the drain so that concrete does not clog WEEP HOLES of drain. These weep holes and
the sloped floor (under membrane) direct any residual water to drain --->>Increases life of pan!
Also.. why no prefab shower floors.. Prefabs are great... if set in modified thinset mortar or similar
(reduces squeeks/movement inherent with prefab shower floors).
Let us know what you think... let us know if need more... Mark
*Still waiting for others to "chime in"...