First off, thank you devs for continuing to expand the game to make it interesting. I'm sure modifying that much of a code-base is a pain. I'm sure you have probably considered just starting the DDO2 project by now considering how large it has become.
With regard to the new EK PrE, I played around with multiple builds on Lammania and ran into the same problem no matter what the build. At level 15 with 18 con base I still had just under 400 hp even when I splashed some fighter levels in there (the most successful build being the PDK 9/6 Sorceror/Fighter build). With spell progression being what it is these days on NPC's, you can still get one-shotted with over 500 hp at that level range. I realize that wiz/sorc have quite a bit of mitigation options such as displacement but I do not think it would be unfair to add a core ability or modify one of the core abilities to grant something similar to the druid nature's warrior PrE where you get one hp/point spent in the tree. I know Stalwart Defender and (drawing a blank on the new pally version) add hp but this is for the people brave enough to attempt the capstone which would be a nearly guaranteed death in late Heroic content and a definite death in Epic content.
On average I am investing 42 AP and that is mostly spent on reducing ASF so that I can no fail with a heavy shield and heavy armor. So far the most successful build has been a PDK self-buffing turtle build with Stalwart Defender. Tenser's plus stalwart stance puts me at an above average AC around 66 with a boost up to 75 using just the PDK starter gear which isn't bad at all considering this is a non-tanking sword and board build. Hold shield-block in an aoe spell such as acid fog and rotate cleave, great cleave, Eldritch Strike and Eldritch Tempest. Magic missile and chain missile for any mobs remaining at a distance. I chose not to splash an evasion class for two reasons, Heavy Armor grants higher AC and PRR and the base saves for wizard/sorc are horrible which would result in few spells actually evaded.
Lastly, perhaps add a feat to the game that trades spell points for extra hit points that could be taken at level one. Maybe make applicable characters choose either Magical Training OR "Arcane Fight Club" (or whatever you want to call it) which changes your hit point progression to that of say a cleric (d8 instead of d4) at the cost of having to pot or find a rest shrine once you run out of SP. This would promote potion sales in the auction house and in your DDO store since EK runs out of SP fairly fast when you have to splash classes. I can easily imagine running out of SP in a raid fight such as Harry. The caption for it could be this:
Arcane Fight Club: While your peers were busy studying and learning to draw energy from the world around them, you were brawling in one of the sub-level arenas to earn money to pay your tuition.
One last tweak that I just thought of. You have designed the PrE for wearing armor and yet include enhancements designed not to wear them or that will be quickly obsolete. Perhaps change them to be simpler such as bonus enhancement to equipped armor and equipped shield as well as granting the secondary bonuses that are in place? Granting the additional enhancement bonus to your shield would further improve the T5 enhancement (would be better if it was calculated based on shield bonus and not enhancement bonus) with more temporary hp solving the above issue with hp. EK's would then have to manually evade attacks until they built up three stacks of temporary hp again making a new play style where they are more stick-and-move point-blank-burst dps.