I think for me, the main point annoyance with something like Frazzle scaling based on the gauge would be that... you're already punished for not having enough gauge going into a 2 minute window, by not being able to put as much damage into burst. Less so now, admittedly, with Manafication changes, but you still lose out on a combo. Being punished even further for lacking gauge - especially if you have no gauge because of a death - feels worse, especially when on something that reduces not only your own DPS but the entire party's buff window in the 2 minute meta. Yeah, currently it's just a "pseudo-cooldown on the Enchanted Combo", but the more you put onto gauge, the more punishing death becomes. I'm not really sure it's necessary to have more punishment for that, even if it leaves the gauge a little boring.
If 2 minute meta was gone and Frazzle wasn't something that would stack with the entire rest of the party's buffs, I think it'd be less bad, but... well, your ideas have all been given assuming that nothing about the combat system is changing, so the 2 minute meta's what we're working with.
Leaving my other comments on Chainspell aside - I think it's hard to not see Chainspell be a displacement of the combo when it costs the same and takes the spot of a combo in your burst window. Obviously the Enchanted Combo itself doesn't go anywhere, but it IS a direct replacement of a combo in burst. Frankly there's not really any way that a new burst would avoid that, since our current burst is just... two melee combos (three, if you work at getting that in under a pot window).
That's a fair point. It would be nice if it was an oGCD like Warden's Pean so it wasn't "directly give up damage", but that also doesn't really fit a caster as well.
Yeaaaah, that's kind of the issue with FFXIV's combat system (to the extent that one considers that an issue, of course; it seems to be by design). More involved support roles would require that kind of support being not only usable in raids, but also at least somewhat on par with bringing roles better at the four things you mentioned. When bosses are immune to pretty much all forms of crowd control, and jobs are being explicitly designed to not really need any other job being present for them to work well (for example, no jobs that naturally run dry on MP in normal gameplay without help from someone else), there's not a lot of room for a support role that isn't failing from the start.