Think you mean CU, not CO. And yeah, I agree with this sentiment, we shouldn't overlook this or simply put trust in SE as to not make huge busting abilities.
Anyway. I did not think of comboing LA + AH (in your rework) would be possible. In that case, it would definitely change and I think AST would be ahead in terms of mana (as long as a SCH can't just lolspam Energy Drain). Going to skip pure balance things for now though.
The lower manacosts were there because SE thought cards would make up for lower potencies, but then they forgot we miss cooldowns ('cause RR/Spread/Shuffle/Sects take up action slots) and actual effective traits like Freecure, Critlo, you name it. The sect bonus for Diurnal I never really understood, I guess it was to emphasize the spothealing powers of AST and make them save mana on chain-casting Benefics but in the current meta you're never in such a situation anyway. The sect bonus for Nocturnal was mostly fairy-compensation, and a low one at that. With your changes, yeah, you could simply ditch the decreased mana costs.
About N.Aspected Benefic: The way I see it, you either want it to be less potent but more mana-efficient than Benefic 2 (with the opposite compared to Benefic), or you want it to do something special that neither Benefics will achieve for you. Right now I believe Aspected Benefic achieves both those effects (live).
If you change Ewer and Bole to those effects, you'd honestly tackle enough. Bole becomes less useless in down-time (still wouldn't really want to use it without taking damage though) and Ewer becomes a really potent Regen card. Arguably too potent but again, let's skip absolute numbers.
As much as I love Arrow, the backlash is what keeps me from wanting it to be buffed further or used more (except for a BLM). The key part is that as long as TP-management should not be a problem in fights (or it seems that way if I look at SE's design), Spire's TP component won't see the light of day unless you overuse Arrow. I'd just tack on an MP/TP consumption reduction on Arrow and call it at that if anything, no need to make the small things too complex when the focus is on switching sects and card draws.
Spear is a bit of a pickle, especially knowing the weird limitations (hurr PS3) SE puts on what can and can't be done. Personally, I'd make spear do both the duration and the stack component with a twist: if no (x amount of) cooldowns are used in the duration of spear, you get a (x amount of) stack(s) of Spear to reduce cooldowns as you use them.
Could your changes work? Yeah, definitely, but it would make the job really beginner-unfriendly.