Just how much Defensive Ability is too much? Just took my first build through Bastion of Chaos (for some reason the char can’t talk to the Master in Crucible) and it took for-fucking-ever with 2994 DA since it was a retaliation build.
I have a character with 2400 DA, and nothing really bothers me. I think you could get away with dropping down to that much. The main thing is to get rid of those damn enemy crits, I think at 2900 DA enemies are missing and critting themselves. 
2400 may be the spot where all enemy crits are removed, but I dunno. I just know I have a 2100 DA character and a 2400 DA, and it’s night and day, because of the crits.
Does retaliation even proc when enemies miss because of your DA? If it’s true, i think you need to lower your DA.
Does Retaliation still only proc on melee hits and ignore spells/ranged attacks?
Short answer: you can’t have enough (with a few notable exceptions, retalliation being the most obvious).
Long answer: before the expansion 2400 was indeed the number to aim for in the main campaign to avoid all crits. Crucible mutators and difficulty levels can combine for some nasty suprises so you’d need more there. After this expansion it’s safe to assume that the avoid-all-crits number is now higher with the increased level cap.
However, your nearly 3000 is certainly overkill in any situation. Impressive though, I never really made it past 2800.
2700 is now the sweet spot, even for Crucible. But even in the Crucible is better to have more than that, maybe 2800.
I dropped DA from 2900 to 2400 on my retaliation build because monsters were missing too often, and missed hits do not trigger retaliation.