I think that your idea of having our abilities extend our shields durations could work if we get a baby TBN introduced at lv 30 (something like a 10 second shield that mitigates 15% worth of our hp) that upgrades to TBN at lv 70, or else there might not be a point to "managing shields" if we don't get it started till lv 70. otherwise we end up in the current state of DRK which is just a blue DPS with some tanking cooldowns and no utility.
The only problem I find is, do we incentivise making long lasting shields and keeping them up, or incentivise using small but powerful shields during specific times that pop to proc Dark Arts/mana refunding?
For Bloodspiller and CnS being multi hits, I think it might be better if Quietus could be the multi hit aoe attack while CnS could be the multi hit single target, which have their effects amplified by your new delirium. as for bloodspiller, if delirium is changed once again to enhance abilities, BS could be changed under new delirium to be a sort of "STB delirium" attack that consumes 50 blood but refunds us enough mana for a TBN, just in case we need to convert resources like Delirium used to, and that way its not a spammable attack, but instead is a big hit with resource conversion that we can use sparingly (maybe every 60-90 seconds, depends on Delirium cooldown)
if Dark Missionary is meant to spread shields, it definitely needs to be introduced before TBN for raid utility. if dark mind only works while you have a shield, your shields might need to be strong enough to survive while you pop the buttons back to back on multiway combat, but in single it can work as a preemptive for a magical tankbuster.
if we generate mana/blood more than the current build, do we introduce abyssal drain as a spender to make up for souleater no longer healing? does dark arts spend mana, or is it on a charge system? would TBN and its baby version refund mana if popped/proc DA, or absorb mana thru hits if we go with consistent shielding, maybe having a blood price effect attached to them that maxes at 3000 mp (since theyll be castable to other members under your suggestion, it wouldnt force DRK in the MT slot like in the past)
our resource generation skills would have to be looked at too. Blood weapon is currently our only resource generation tool, so would it be better to shorten the cooldown or strengthen its effects and duration? would stalwart still work the same as it does now? do our shields have blood price effects added to them, or would that make them too strong in terms of dps via resource generation?
I love your LD idea though. having it proc when our HP drops to 1, but converting all subsequent damage into a shield that maxes at 50% would be a cool way to change the skill, while still keeping it different from the other invulns. I would say that change alone would make LD much better than its current iteration by a lightyear.
TL;DR
the way i see it, we either go with short powerful shields and have Dark Arts remain as a proc for TBN which affects multiple abilities, or we go with the sustained shield model, which means DA would have to return either as a spender or charge ability. introducing Shadowskin at lv 30 (15% hp shield, 1500mp cost, 10 sec duration, 30s recast) would be needed for either build, since it could either be sustained or refund mana if popped.
if we go with short shields, I would say let Dark Mind gives us 20% magic dmg reduction and generate its own shield that is worth 10% hp. otherwise your idea works for sustained shields model.
Delirium could enhance Bloodspiller to generate a large chunk of mana on usage, make quietus and CnS have multihits while retaining their innate mana generation. maybe even enhance stalwart to generate extra resources per hit.
Dark arts could give free Edge/Flood/Abyss Drain, could possibly give CnS/quietus a potency boost with no mana generation, could give shadow wall the blood price effect (for MTing) or maybe could give Dark Missionary a blood price effect (for OTing). blood price effects would have to cap at 2000-3000mp to not be dps advantage over the free spenders. if its on a charge system, we could make it grant the blood price effect to our TBN instead of shadow wall.
Blood weapon, Stalwart, and Syphon would remain our standard mana building tools, with CnS and Quietus having inherit mana generation which are enhanced by Delirium, or removed for potency buffs by DA. Salted Earth should return as a blood builder as well.
Living Shadow could definitely be our burst source, perhaps esteem mimics our rotation to output extra damage.
Your LD idea is pretty bomb, so we keep that one regardless of which build we go with. much better than square's version.
Lastly, I would change souleater into power slash, but that's just a personal choice cuz I think the skill has a sick animation.