I liked your post. I am continually learning new things. I wanted to ask a question about battle awareness.
I think that the armor added is to each piece of armor. So it is a larger percentage of 350 (cuffs) than it is of the total armor you are wearing. Also try playing an act with armor 80 lower (or 40 and 20 in epic/normal respectively; in each location) than is available from the merchants. You take more damage from mobs. So I think this helps. For the defensive ability you have to remember that it is a relative number. You compare mobs offense and your defense. So in legendary you might have 1200 DA and a small boost seems unimportant. But the mobs have 700 OA and that small boost does give you a little edge. But I admit the DA is not good. Mostly I think the armor is good. It is like having free Legendary boars hide on every piece of armor with completion bonus of tiny DA. (Edit: my DA argument boils down to that while its a small % of final DA the effect of DA is NOT simply the magnitude of DA but the differential between monster OA and your DA. The problem is that monsters keep getting more OA.)
Also I am of the opinion that the survival instinct is pretty important in legendary. In the later acts it keeps them at slivers of life for prolonged time. For example it sucks when manticore stops attacking your wolves because he killed them and then he turns and bites your head off!
Also some skills are worth maxing in normal. For example squall does a lot of damage relative to normal for 12 pts, covers the map, neutralizes the most annoying enemies (archers). Another example is DW. You want a shield rather than 1 pt in each DW skill. If you DW better max some procs even though you get the most % for the 1st point.