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.
Source: Celestial Storm FAQ (2018-07-20), TPCi Rules Team (2018-08-02)