Thoughts:
Mass Hysteria used to increase DOT damage by 2% per second while in Voidform, not all Shadow damage, just an FYI.
Regarding Lingering Insanity and Mass Hysteria, I think we need to move away from these long ramp up mechanics entirely. They are a really unhealthy design for the spec and the game. Almost all combat ends in less than a minute, and Shadow is still ramping up in that time due to Mass Hysteria & Lingering Insanity.
Fortress of the Mind should definitely be deleted. But I think the biggest issue on that tier is that 2 charges on Mind Blast is mandatory to play the spec properly - there is nothing they can add to that tier currently (due to the importance of not delaying your Mind Blasts for insanity generation) that will compete with 2 charges. So they need to delete Fortress of the Mind, but they also really need to make 2 charges on Mind Blast baseline.
Chain Mind Flay sounds super cool, I'd love to see that - but Mind Flay would have to do actual damage for it to ever be worth casting. Since all our power budget is in Voidform and ramp up mechanics, all our other casts only matter if they generate Insanity: which Mind Flay doesn't. So if we don't delete Voidform/Insanity, then even an empowered Chain Mind Flay would still get clipped into a single GCD to fill between Mind Blast & Void Bolts.
As example, Void Torrent isn't worth casting for the same reason- even though it does 3 times the DPS of Mind Flay. In Legion Void Torrent was amazing because it paused Insanity loss, and allowed you to gain Insanity during it. So when you got to high stacks (ex. say 45 currently, where I usually drop out of Voidform), you would use Void Torrent to extend it for another 4 seconds: to 49 stacks. Since it doesn't pause Insanity loss anymore, it's garbage. Oh also, Legion Void Torrent did more damage and had a shorter cooldown than the talent, and was baseline - so it didn't have the opportunity cost of being a talent. Sorry, just a little rant about how absolutely they wrecked Void Torrent in BFA
Mindbender really needs a way to distinguish itself from Shadowfiend. The DOT idea is interesting but if you're currently multidotting properly, and using Void Bolt properly, your proposed change wouldn't do anything - those adds should already be dotted if they are worth dotting, and Void Bolt will extend dots if they are dotted. What I'd like to see Mindbender do is something like channel an empowered Mind Sear for the duration. It's a Mindbender after all, it should do some more Mindy stuff rather than just whacking it with tentacles.
I like the idea of where you are taking Dark Ascension - it obviously needs to be fleshed out more - but if they gave us the old Dark Archangel wins during it, that would be super cool!
I really want Spectral Guise back for PVE reasons too - a big part of the allure of Shadow in like Pandaria/WOD, was that we could ghost mechanics - the same way that Hunters Feign out of targeted mechanics, rogues stealth, etc. Spriests used to be able to Guise mechanics - and that was amazing utility to nullify the occasional mechanic like that - and there are like at least 12 other specs that can do it, not to mention all Night Elves (Shadowmeld).
I liked what they did with the Mind Flay PVP talent that steals haste, but I think it would be cool if the Mind Flay slow got stronger every second it was channeled. So start it at a 50% slow, increasing 10% per second, with like a 4 second duration (so we can weave in a dot refresh or mind blast or etc). So you can potentially build to a strong slow or root even.