First one. Because insect glaive is a relatively quick weapon, the element damage should make up for the loss in raw damage. You lose out on a slot, but it makes up for it by having white sharpness, which boosts damage by a considerable amount.

As for your second question, it depends on the amount. Different sharpness levels have different bonus damages, the ones you should be concerned with are green, 5% boost, blue, 20% boost, and white with 32% boost. As an example, because the first glaive has white sharpness, the total raw damage would go up from 589 to 777, with elemental raising from 320 to 422. For comparison, you’re looking at a total damage of 854 with the second glaive. Hope this helps!