Flat damage would scale with any leech applied from your weapon damage. as Fire Strike has over 100% by default, you would always get the full impact of leech.
Explosive Strike is a secondary effect that creates an explosion around your target. As it does have % Weapon damage, any damage it deals will leech proportionally to the % Weapon damage it has. Any bonuses from the base skill are not included.
Static Strike is as vanilla of a modifier as they come when it comes to calculating damage/leech.
Brimstone is a secondary effect that launches fragments at nearby enemies. It has no interaction with the base damage of Fire Strike. Each fragment only deals the damage listed on the tooltip. As it has no % Weapon damage, it does not leech.