Silcoon

After Roggenrola uses its "Harden" attack, is the prevented damage amount calculated before or after Weakness/Resistance?

Weakness & Resistance are factored in before determining whether Harden blocks it or not. So if a Grass Pokemon's attack does 30 damage to Roggenrola and then +30 more is added for Roggenrola's Weakness, Harden would not prevent the damage. However, if the Grass Pokemon's attack does 20 damage to Roggenrola and then +20 more is added for Roggenrola's Weakness, Harden would prevent the damage.

If I use Roggenrola's "Harden" attack and my opponent switches it to my bench, can I prevent any bench damage of 40 or less done to Roggenrola from my opponent's attacks?

No, once Roggenrola leaves the Active Spot the effect of Harden goes away.

If I use Seedot's "Harden" attack which prevents any attack damage that is 40 or less during the opponent's next turn, does that come before or after calculating for Weakness/Resistance?

Weakness & Resistance are factored in before determining whether Harden blocks it or not. So if a fire Pokemon's attack does 30 damage to Seedot and then +30 more is added for Seedot's Weakness, Harden would not prevent the damage. However, if the fire Pokemon's attack does 20 damage to Seedot and then +20 more is added for Seedot's Weakness, Harden would prevent the damage.