I disagree with your analysis of the problem.
At the heart of it, the problem with the 2E Abyssals is that the only unique story they have is "Redemption", and "my story arc is that I want to stop playing this splat" is hardly the most alluring of things. Beyond that, they're basically gothy Solars who unlike Solars, by default have uber-powerful nigh-invincible ghosts who you can't kill permanently and have all the same Charms as you as your bosses, telling you what to do. Their Charms are Solar-Charms-but-Worse, Resonance is just more painful than Limit, and so they essentially fail to provide a unique playing experience.
So your solution to Abyssals not having a unique story is to force them to tell your particular unique story only? Infernals are allowed to ditch the Reclamation, there's Dragonbloods who aren't part of the Realm, and even a Sidereal can elect to wander the earth and kung-fu bandits instead of politicking in Heaven.
I can agree that the Passion-driven Revenant who'll go back to being dead once his revenge is fulfilled and he's happily back with his wife is a valid story concept and should be playable. But I disagree that it should be the only valid story concept.
The rest of this quote can be rephrased as the Writers deciding that Abyssals should have the two aspects
- Abyssals as evil Solars, so all their charms have to be inverted Solar charms, and if you want to play a good Abyssal you have to play a Solar
- Abyssals as loyal bootlicker to the Deathlords and doom dispensers incapable of anything besides killing who don't get to be happy
and those aspects only, and writing them accordingly. Which is exactly the narrow and specific vision I referred to, so why do you say you're disagreeing with it?
Now, of course, there's no solution in making Abyssals just feel like Infernals - hence why I've been against having Essence-based or Principle-based charms for Abyssals. But still, the point remains - limitations define a splat, as do the intended stories you're meant to tell with them. And if a person wants a splat that plays like a Solar and tells Solar stories, they should be playing a Solar. Because that's what Solars are there for.
Chasing a measure of happiness despite all the obstacles your very existence throws at you - evil omens and absolutely terrifying powers making people afraid and suspicious, catastrophe following whereever you go, and always the temptation to just stop playing nice and start killing, and with enough effort just maybe succeeding for a time, is an exclusively Solar story now? No. Come on. Now you're just repeating Abyssals 2E, "if you want to play an Abyssal who isn't a murderous asshat, go play a Solar," when lines ago you spoke out against it.
Solars absolutely can perform sorcerous mutation and artifact cybernetics on themselves, despite their themes being human excellence, and cybernetics and mutation being the nominal domain of Alchemicals, Lunars, and Infernals. Sidereal martial artist wearing an appropriate fate mask get to play the wandering hero just as much as Solars do, even if they won't have it quite as easy. Likewise for Infernals, and nobody will force them to take the Pantheon charms if they don't want to. Shapeshifting is the Lunar's shtick, but Abyssals should still get to turn into the Grim, even Infernals can shapeshift to an extent with the Devil-Tyrant Avatar or maybe some kind of heretical Coadjutor shintai; They won't be as good as a Lunar, but they can still do it, and worst-case Sorcery still lets you turn into a flock of birds. A Solar who exalts after he decides to make up for a failure will have the exact same theme of Second Chances as an Infernal has by default. People other than Abyssals can learn Necromancy. Solars with high permalimit don't actually pull off the Cursed Hero figure any better than successsful Renegade Abyssals should.
A certain overlap will always be there, and that's absolutely fine. Themes provide emphasis, inspiration for character concepts and lenses to filter them through, they are not absolute constraints.
You say thematic purity, I say the notion that it ever has existed in as rigorous a form as you're presenting is as bullshit as the notion that it should.
I think the bigger issue is that rather than Lunars, who have one mechanical implementation, this would seem to require several different implementations all working off of similar but not the same flavor.
Not really?
I mean, Bob McMurder is just the Abyssal Melee tree, as well as the fact that most Abyssal charms and Necromancy spells should be ab-/usable in various horrific and/or hilarious ways.
Turning into a Solar, whatever, it's an option for those who want it, when it happens cash in all your old charms and buy new Solar charms that fit.
The Passion-based Revenant can be done by Abyssals being undying only as long as they choose to be to accompany Abyssals exalting when they die, the rest can be done by characterization and maybe the occasional optional Must Get Revenge Integrity charm.
Optional other charm trees with adjustable FSX and base skill can account for any appropriate flavor of shapeshifting, raising the dead either by commanding them to or by beckoning them with fluteplay, and so on.
The 'temporary happiness if you struggle for it hard enough' can be done by modified Resonance being an on-demand Plot Hook dispenser, from people going mad in your presence to bloody hurricanes striking your position to lynchmobs forming because holy symbols catch fire at your touch.
Overall, shouldn't be more difficult/have more cdifferent sets of implementation than writing Infernal charms. Which granted, I still couldn't do, but other people do.