I'm going to respond to this as a single post, since they're getting kind of clumpy.
The main problem with dual catalysts not being viable isn't anything to do with the spells themselves, but with the mechanics of using a catalyst. It's much like trying to use two weapons in DaS3 when they aren't paired weapons. You can do it, but you don't get any particular advantage from it and you lose out on half the moveset of one of them. Likewise, using two catalysts isn't any different because you can still only use one at a time, with the only difference being that you don't have to switch them out one at a time but you give up a weapon and/or shield. Both the casting mechanics and the lock-on mechanics conspire to ruin pure caster setups, so giving up a melee weapon is tantamount to suicide. You can compromise, though. Get one of the catalyst weapons in one hand and a normal or hybrid catalyst in the other, like Demon's Scar and Crystal Chime or Golden Ritual Spear and White Hair Talisman. Your spells will be weaker, but being able to just hit people when they get too close is so worth it. Too bad there isn't anything like a paired catalyst, either two staves to dual-cast sorceries (either supercharged or just two at the same time), or a dual staff+chime for dark spells or sorcery and miracles.
I disagree with White Hair Talisman getting Unfaltering Prayer, mainly because pyromancy being uninterruptible isn't negated by pyromancy already being fast. Chaos Bed Vestiges or Black Flame being able to trade instead of having to worry about spacing or rollcatching is horrifying to think about. Also, White Hair losing Combustion also loses a bunch of combo potential like Lightning Stake followed by Combustion. Really though, White Hair's real issue is that it does a full 50% less damage than a normal pyro flame or Yorshka's Chime, whereas Crystal Chime only does 20% less damage than Court Sorcerer or Yorshka's. Speaking of Chaos Storm and UP, I seem to recall that one of the recent patches gave it poise? I haven't checked, but it doesn't seem to have made a difference when people can still circle around and backstab you. The spell (and all others like it) is and will remain terrible up until they let it multihit again, and then it will be broken again. It's either "one-shot the boss" strong or "not remotely worth the cost and time of casting" strong, there's no in between. On that note, Divine Pillars of Light would be awesome if it only staggered instead of knocked down, so someone trying to roll through it for an easy backstab could end up eating multiple pillars worth of mediocre damage instead of one hit.
I can't stress Sunlight Spear enough, really. It has a fast casting speed (don't know what you're trying to say about it being slow), the animation looks very much like Lightning Stake which is good for mindgames, and so on and so on. It's like an inverse Great Soul Arrow, where literally the only bad part is the FP cost and everything else is fantastic. Though, if you think GSA is good, that would at least explain why you think Sunlight Spear is terrible when it really, really isn't. At all. And the damage is huge, by the way, when you equip for it correctly. I don't think any amount of words could change your mind though, and I get the feeling I'd have to show you in-game to get the message across. When I say "Sunlight Spear", you're thinking of something that better fits your biases rather than the actual spell itself, but you've already said you're aware of that.
As to having different spells for specific builds, I'm actually okay with this and prefer it. I'd much rather different builds, like hybrids or lower level ones and so on, should have access to completely different spells with different uses rather than all possible builds getting the same spells with the same uses. I hate it when different builds all feel the same even though their stats are wildly different, which is why I hated DaS3 PvP for literally a year before the patches and DLC started to introduce something resembling build variety. And now, about 365 days later, two wonderful things happened: A) dark spells now have a legitimate use instead of being strictly worse than everything else despite the massive stat investments, and B) hybrids now have a category of magic all their own to further distinguish them from dedicated casters or even pyromancers. And the answer to both of those questions is the same thing, Murky Long Staff, which also gives a surprising insight to the Sunless Talisman that nobody noticed before, namely that it can do the same thing for dark miracles that Murky does for dark sorceries. All told, I'm happy with things as they are now.
Setup spells are... tricky, to say the least. If they're visibly distracting, you run the risk of them not being actually distracting (if an experienced player can just be not distracted by the lightshow), and there's the annoyance factor of another player blocking out your screen and potentially killing you by way of interface screw, which is a pretty ******* way to win. If the setup spell is too good, then you run the risk of it working every time and always winning. If it's not good enough, it becomes literally useless. If the setup spell itself is good but the followups are too slow or weak to take advantage of it, then you have the problem where the setup spell is technically useless despite being arguably perfect. Black Serpent, after numerous patches, now neatly qualifies as good enough and is edging just a little bit close to too good (side note, try blocking Black Serpent instead of rolling through it). Making it any better risks becoming a problem, though I'll fully agree that other spells like Floating Chaos or Profaned Flame need a bit of a boost. It's such that your only real options for serious caster pvp are omnicasters or hybrids, because pure casters just lack the necessary tools in one or more areas. For Affinity, I wouldn't say boost its tracking but rather boost its projectile lifespan. In previous games, Affinity's big threat wasn't that it chased you near perfectly (it actually didn't) but that it lingered just long enough to circle around for a second pass if it missed the first. Kind of like Corona, actually. Black Serpent is kind of like the faster, briefer version of DaS1 Pursuers, while Pursuers/Affinity were the looming threats that would kill you dead if you forgot about them.
Fire Whip: I haven't tried this exact thing myself, but I've done similar things and it should work, but try using Demon's Scar and doing R1, FR1 (backflip slice), and Fire Whip. If I'm correct, that should stun your opponent and give you enough room to safely cast Fire Whip.
Emit Force: What you should do is toss something slow at them, then fire Emit Force where you think they're going to dodge. If you guessed correctly, it'll hit them and they'll be flat on the floor, waiting for you to try yet another rollcatch.
Great Soul Arrow: Maybe I've just been using sorcerers long enough to not be fooled by this sort of thing, but the fact remains that there are better, more specialized spells you could be using to get an advantage rather than hoping your opponent will fall for an old trick like that. The problem with this sort of strategy is it relies on your opponent making mistakes, which means if they're flawless then they simply beat you without any trouble. Sunlight Spear would be better at this than what you're trying to use GSA for anyways, since it flies faster meaning there's less time to react and the windup looks very similar to Lightning Stake.