I know you need SH2 on for DW poison effect to be AOE, but what actually determines if it hits a mob? Is it SH2 mod vs weapon skill, DW mod vs res, SH2 vs res?

I think its sh2 vs weapon skill and dw vs res.

SH2 on/off turns the DW poison spell from single hit to AOE hit and back.

The poison spell's power combats against the target's resistance.
The poison power comes from the mod DW skill power.