On Part IV:
Dark knights lack physical mitigation severely, being the magic defense tank doesn't really do anything for us when paladin and warrior do just fine against magic attacks anyways. For dark mind it should be 15% vuln down and with dark arts +15% magic damage reduction on top of that. We should not have our party mitigation stacked with our personal mitigation skill, paladins/warriors is not. Either we need a new skill for it or SE needs to turn one of our mostly garbage skills into it like they did with shake it off, (looking at you delirium and soul survivor).
Shadow wall should be the dark knight equivalent of Vengeance for warriors. 20% mitigation for a 15 seconds. Instead of dealing a 50 potency attack it can drain HP from attackers for 25 potency. Dark arts 30% mitigation 20 seconds and 50 drain potency or something. Basically just making it all around stronger. Cooldown needs to be lowered to 120 seconds.
Adding dark arts affects to living dead will create dangerous scenarios in which you want to use dark arts before it on a skill that already procs late for emergencies due to server ticking and will end up getting us killed more often by the attack we were trying to prevent even though the animation went off. There's no reason to make this skill so detrimental when paladins and warriors functions so much more easily. At the "very least" SE could just make it so we come out of living dead with weakness status affect if not healed to full, give us +% healing during it, any number of other solutions if they don't want to change it fundamentally.
Part V:
I read this a few more times and it grew on me. The only thing I could add to this is that maybe despair should be a +% parry chance per stack on top of your listed affect in that we are becoming less concerned with dps and more concerned with our survival. Parry could also trigger a % chance of plunge cooldown reset giving dark knights back its parry proc dps. It would fit well with your listed plunge change too even make it possibly more weave lenient.
Something worth noting: Its been mentioned that dark knights could just as easily have TBN be AOEable for 10% with use of dark arts. This could work well as our party mitigator instead of using soul survivor or delirium, they could be altered instead to become a personal mitigation tool or stronger DPS tool. I feel like DRK should have more HP consuming/draining capabilities since that has been a part of its lore for as long as I can remember.
Edit: I also wanted to add that I firmly believe soul survivor should become a form of Instant HP grab with a cooldown of 1 minute instead of this compile thing. As it stands, were the only tank class that cannot heal itself in an emergency unless souleater is next on your combo list which doesn't seem fair...