The direct hit damage still does not benefit from Double Tap. That said, I am glad the AoE does, thank you.
Further, the buff snapshots when swapping forms, restoring when you swap back, even if the value is lower. For example, stack normal form so the buff shows 140%, swap to incarnon and stack to 500%, then if you swap back, you lose the 500%, and get set back to 140%. If you then swap back to incarnon, you go back to 500%.
Note: This requires mods so that reload speed is faster than the Double Tap decay time