- No, the additional dmg by imbue skill like Envenom Weapon, Earth Enchanment or Storm Nimbus don't work with skill like Fireball, Eruption,...
- Yes, Str is affect the dmg output better than Dexterity before Anniversary Edition version on pc.
To add / clarify.
Flat damage bonuses will not affect your skills, but % bonuses like that on Fire Enchantment will.
So 5-8 Fire Damage (no), +100% Fire Damage (yes), +100% Burn Damage (yes).
Another example
Nightshade +50% Poison Damage (yes), 39% Slowed (no)
The slow on the Envenom Weapon tree along with all it's other effects will not apply to your skills, only your weapon attacks unless it is % damage based.
If you just want an AoE Slow, 1 point in Poison Bomb will get you what you need. If you want AoE disruption, Flash Powder.