Bolt of Molten Scoria:
Recast Time: 5.75
Cast Time: 3.5 cast time.
Total Time: 9.25
Mana per cast: 989
Mana Per Cast / Total Time = Mana Per Second: 106.918
Damage: 5166
Damage / Total Time = Damage Per Second (unmodified): 558.486
Damage / Mana = 5.223
Spear of Ro:
Recast Time: 1.5
Cast Time: 7
Total Time: 8.5
Mana per cast: 684
Mana Per Cast / Total Time = Mana Per Second: 80.47058823529412
Damage: 3119
Damage / Total Time = Damage Per Second (unmodified): 366.9411764705882
Damage / Mana = 4.559
Looks like you're not wrong.
Perhaps it's because Spear of line goes from level 70 to level 90 without an upgrade and it's a byproduct of a branch of the game where things are out of sync. Because picture this.
Level 120 mage.
Bolt of Molten Olivine - Level 116 - Highest Bolt at 120
Spear of Molten Luclinite - Level 120 - Highest Spear of at 120.
Bolt of Molten Olivine:
Recast Time: 6
Cast Time: 3
Total Time: 9
Mana per cast: 3289
Mana Per Cast / Total Time = Mana Per Second: 365.444
Damage: 38123
Damage / Total Time = Damage Per Second (unmodified): 4,235.888
Damage / Mana = 11.591
Spear of Molten Luclinite:
Recast Time: 9
Cast Time: 3.5
Total Time: 12.5
Mana per cast: 8962
Mana Per Cast / Total Time = Mana Per Second: 716.96
Damage: 73014
Damage / Total Time = Damage Per Second (unmodified): 5,841.12
Damage / Mana = 8.147
The clear winner for raw damage is Spear of Molten Luclinite. In your case you're comparing a level 80 spell to a level 70 spell. They are not inline with each other. So I see where there is a shortcoming.
There is a similar gap in MQ2Enchanter where breathing dot and ChromaNuke differ and we just figure out which one has more dmg and use that.
Suppose we could do something similar.

So this is the spell lineup that makes more sense?
I can make this happen while still making it automagically load

at higher levels np.