1. ABS is 2"
2. I'm going to remove the old P-trap from the vent and cap it in order to reutilize the vent for the new shower
3. Can I utilize a 90 degree turn in the waste run.
You can have 135 degrees of change after the vent.
4. Where the waste pipe meets the vent, do I need to use a T or a Y or does it matter?
Most codes like a wye fitting in case someone tries to wet vent later on.
5. In order to get the slope I need, it seems to me that the pipe gets cemented in at a slight angle, is that correct?
Fittings are made with that angle built in. 2%
6. When installing the P-trap, does it rest on the drywall from the downstairs ceiling or is it slightly raised, not sure at what depth to seat the P-trap in relation to the joist and subfloor.
Once you have set the p-trap height, it drains down at a 2% slope toward the vent. No..........You can't monkey around with fittings and change that!
7. What's best practice when installing the drain, do I cement a piece of pipe prior to subfloor install and cut after subfloor goes down or do I cement in drain pipe after subfloor.
That depends on what you are using for a shower pan.