Replacing Shower Drain Head Body

I had a leaking problem at the clamping flange in a shower drain. I decided to cut out the slab around the drain head. Approximately 12" x 12" box down to the dirt, exposing the drain head and trap. I scored and snapped off the original drain head, about 40 years old! I want to replace it with a newer style with a rubber gasket, cement in and then put in a shower pan. Questions are:
1) the riser pipe that ties into the trap is threaded on top (outside of pipe)? Why? Should I change this out with a pipe that is solid up to the top and slip the new drain body on it? Think it is galvanized??
2) in terms of cementing in. Do I just build dirt up to cover the trap so I do not pour cement directly on trap or will that hurt it? Notice that the slab seems to be about 1" higher than the trap. Maybe the dirt just settled over the years? Do I need to wrap the pipe with 20Mil tape first?
3) do I need to use cement lath in the box section?
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