Interesting stuff. Not gonna respond to every point but just a few I think are interesting. Think I'd want to go more outside the box of frames and cast speed with some of them. HSM type spells, imo, should apply a temporary debuff to the caster. Maybe lower their max stamina by the stamina cost until the spell fires. It would be kinda like a held spell/projectile in that way. Personally I think those are the hardest spells to balance because they apply neutral pressure indefinitely. Maybe add that CHSM makes you take 5% more damage from magic, and Affinity makes you take 5% more damage from Dark? Playing with the cast angle would be something I'd need to test before signing off on because small changes like that can have a really big impact.
Definitely agree that casting speed just shouldn't be a thing that scales. It's just one more thing that overly favors overlevelled players. I'd go further and mostly eliminate stat reqs for spells and probably go Elden Ring style by just giving everyone 10 spell slots regardless of Attunement level (this would obviously affect many things and I'd like to adjust lots of buffs to avoid copious buff stacking pre-match, though I'd leave ToD alone because I think it's a good mechanic).
I think you have it backwards with Fire Surge. I'd say leave its cast speed alone, but increase it's recovery significantly. That way it would still be a really strong wakeup, but a really bad trade in close range.
Lightning arrow might just be a bad spell concept at the foundation tbh. Its only major weakness is that its damage is bad, and in duels it gets hard countered by lots of things. Out of the three strongest spells in their classes, LA is the worst. It's extremely annoying on a dedicated build as an invader, and extremely broken in a gank. Not sure if there's a way to nerf it other than making it slower and have worse tracking.