Must I use gravel around a new 2" shower drain before filling trench w/ concrete or is the clay soil I removed from the trench adequate, providing I pack well? There will be a minimum of 4" of concrete over the 2" PVC drain line. Is it necessary to replace plastic membrane before concrete is added? It's a fairly dry location and will be covered w/ ceramic tile. Thanks...

Installing plastic membrane in such a confined area will not do too much since you won't be able to water seal it with the existing vapor barrier. Just fill it with concrete and you should be fine.

Do you have a radon relief system in your house? They usually have plastic membrane under the slab if you do. Not that I would worry about it.