Moving a toilet, tying vent to old stack under subfloor or in attic

I’m moving a toilet in a bathroom. It’s essentially just sliding down the main sewer line. The original vent is a 5” cast iron pipe that exits directly through roof. The vanity and tub originally tie into this stack in the attic.

I have three questions:

1. In Option A, Can I tie the toilet to this vent under the subfloor and just re-use it so as not to cut vents into the plates? This would create a pretty long horizontal stretch not higher than the toilet itself which seems wrong.
2. In Option B, I just bring the toilet vent straight up into the attic and tie it into the 5” stack there. Plenty high above the toilet but more cutting. If this is the right move, do I need to keep the original 5” vent connected to the main sewer line for some reason?
3. If I leave the original vent, can I have the toilet not have its own stack since there is a vent upstream connected to the main sewer line?

I’m in SoCal.

Thanks!