Well, after starting a new character (a mage this time rather than a warrior), editing the save to jump to level 25, reloading, learning level 1 of Hell Spikes and then saving again, I used that save to learn up to level 5 normally and with the editor. I ended up with more differences between the skill sections of the data.000 file this way than I did previously.
In a copy of my level 1 character save I tried replacing the entire skills section of the data.000 file from the character level 25 save after having learned Hell Spikes to level 5, but upon loading the save, casting the spell only resulted in a single set of spikes. One interesting side effect, though, was that using Hell Spikes did not drop mana at all.
With no easy/practical way to edit a save to get Hell Spikes working properly, your should use the save editor to unlearn that spell so it can be learned correctly as you reach the level requirements. You can use the editor to jump to level 25 (set your experience points at or above 5,200,000 then reload the save and defeat an opponent or complete a quest to gain some experience and trigger the level update).
Alternately, you could use another spell for now, and keep an eye out for a Hell Spikes spellbook that you could duplicate.
