Sing: Currently sing multi targets 3 spells that normally can only single target, Curse, Hold, and Allure (lamia version), (and toad if he is in toad status) in addition to casting Sleep as the replacement for the 'nothing happened' spell. Since Edward will now learn full white magic this may be subject to change however, they seem to be decent upgrades to the original single target Sing command that only cast: Sleep, Mute, Charm and "nothing happened". (im open to suggestions)
For the curse song's name "Song of Sadness", IMHO its kind of misleading (I played too much Earthbound) and there are better words that could be used instead. In the recent versions of FFIV, there is a enemy attack called "Cursed Elegy" (Milon Z/Scarmiglione - Zombie Form). If you are looking for a more original name, there is always Cursed Song, Mysterious Elegy, Mysterious Melody, Cursed Elegy, Eerie Song, Song of Despair, etc... For the visual, use Milon Z's DullSong/Cursed Elegy effect (it's the sleep spell effect, but its brownish instead of bluish/purplish) and it has a different sound effect (Pollen sound effect if I remember correctly).
For the "nothing happened" failure song, I always imagine that a failure song would be a song that would deal damage to a single target/multiple targets instead of inflicting a status ailment. What I mean is that if you listen to someone playing a song and he screws up the notes or he's a bad singer, etc. To listen to him, he'll hurt your ears due to the bad noise/bad notes played/etc. Discord Song (what I called it) would have the power of a level 1 spell (Fire1, Ice-1, Lit-1) or at best would have the power of a level 2 spell (Fire2, Ice-2, Lit-2). The power would increase as you level up, so it would be tied to your Wisdom or was it Intelligence (anyways, the one that is the black magic stat). Discord Song would have a non-elemental property or have the darkness property. For the visual, I would used the Dispel/Dummy effect.
Edward's two other songs are both fine (Song of Charm and Stunning Melody)
The white spell "Nova" ("custom spell, like a "Holy" version of meteo slightly weaker because it has a specific element"). The spell effect I suggest to use for your Nova would be Last Arm's Fission because it looks more like a nova/supernova. Meteo spell effect doesn't look much like a nova. Or, how about you simply rename Nova to Comet and rename the twincast "Comet" to Stardust instead. Comet is usually the weaker version of Meteo/Meteor.
"Blast (Cid) - casts "Fission" on all enemies, killing the caster." Since this game will be a little harder than the original, how about replacing Cid's Blast with Magnet (inflict stop status ailment), but for Cid make it able to fail too. Magnet seems more like a ability associated with Engineers than Blast IMHO. I remember some machines used this technique in the Giant of Babil.
Darkness element, does that mean that Cecil and Golbez's sword (they are Dark Knights) will all/majority have the Darkness element associated with them? In the original game, there weren't many enemies or any at all that were weak against Darkness. I know there were some that were resistant to it. I am hoping that some enemies will become weak to the darkness element and/or your patch will introduce the darkness element.
It's sad to hear that Sheila has been discontinued/removed, and yes I like Yang and his daughter Ursula. So what will become of Sheila's sprites? I suggest you summit it to sprite-ressource or something. It'll be such a waste to let her spritesheet be deleted.
As always, just tossing ideas your way and of course keep up the good work Zozma. I am waiting patiently for the patch. Good luck.
Add: Here is another idea that just came to me, how about using Zeromus (aka Zeromus EG) sprite and battle script from FFIV - Easy Type (snes) or Gameboy Advance.