Woah what a thread i have missed, damn!
Not gonna read everything, just wanna drop my point here.
The problem isn't the regen itself (frankly its quite weak), the problem is how powerful the defence stacking is, and how easily some classes can do it without sacrificing in other aspects like damage.
Armor and avoidances stack additively which results in
hyperbolic growth of their efficiency - the more you get the more effective your investments are.
Even the current hard cap of 75% doesn't help much coz 75% is still very very high: thats a 400% increase in the amount of dmg you can survive. And if you have both armor and for example parry at those 75% values (reachable for Chosen, BG) then thats 1600% increase in the amount of damage you can withstand.
Imagine these values, but as damage buffs instead, sounds completely insane, right?
This is because armor/avoidance/toughness
reduces the amount of dmg/hits that lands on you, so the closer you get to reducing it to 0% the more efficient it is.
To demonstrate with an example - going from 0 parrry to 10% parry means on average you'll be able to tank 11% more damage than HP you have, thats called Effective HP (you'll be hit by only 90% of all attacks, so 1 / 0.9 = 1.11).
However getting the same 10% parry when you already have 50% (which is alreay 200% Effective HP) will take you from 200% EHP to 250% EHP, which is 25% more and lets you survive a whole extra half of your base HP as damage (1/0.5 = 2 = 200% ehp, while 1 / 0.4 = 2.5 = 250% ehp, 250/200 = 1.25).
This sort of math applies to all damage mitigation values that stack additively in this game, but most noticeable with Armor and Parry/Dodge/Disrupt/Block due to them being easily stackable without softcaps and high fairly high hardcaps.
Trivial Blows is also affected by this, which makes its low values very ineffective, while going all 4lvls in it is good (first 3 levels give you 24% reduction = +31% ehp vs crit dmg at the cost of 30rr, while 4th level boosts it to 40% reduction = +66% ehp at the cost of only 15rr)
Damage bonuses like crit or strenth, on the other hand, have
opposite growth effect as you stack them - each further point of investment is relatively less effective because you already got a bunch (although the effect is fairly subtle due to low values).
In other words its better to build multiple sources of damage, instead of relying on maxing out a single one.
Until this disparity is addressed there's no point nerfing Regen, becuse it will just kill it for non-deftard builds, while still leaving the true cause of the problem unattended.