Hm, I could see them making certain skills have an individual hit crit chance, like RoV, or the Wizards Meteor that splits into 13, and then have some like Multishot that don't, but it would be more complicated then needed, but for now I'd assumet hat you would be able to crit on each individual enemy hit by. An easy way to test it would be to see if the Amazon's multishot could crit on individuals, which is probably tested already, most likely they will keep the same sort of mechanics from that.
ANY build can probably get through nightmare, unless they really do something about the scaling of it compared to D2, which is certainly possible. But if your like me and do full explores, then you will be a much higher level then the scaling is set for. They probably scale it for a decent amount of exploration, but not full complete explorations of every area. I know that by the time I hit NM in D2, I was stupid over powered, because I was a good 4-5 levels higher then the content, and stayed that way throughout the difficulty, which means you would also continue that trend through Hell. Inferno will be a whole other story.
I have kinda adopted the same sort of view on builds. I'm going to try everything out as I get them without runestones, and then as I get runestones use them in skills that provide the most benefit for the current things I'm doing. When doing Inferno, I'm hoping to have stock piled a good amount of them, so that I can switch things up as I go, if I enter an area with a bunch of Cold resistances for instance, I'll re do my build to account for that. Since you can just mouse over an enemy to see it's resistance this can pretty easily be done. If I then encounter fire resistant monsters in the next area, change it up again.
Honestly I don't think in the higher difficulties Brooding will be very worth while, it just seems very unlikely that you will be able to avoid taking damage enough to make it have a decent enough up time to be worth the passive slot.
What kind of weapon are you planning on using, because that heavily dictates if Archery will be worth it.
I LOVE vengeance, which I might have said above, but it all depends on play style, but the Hatred increase alone is good enough reason for some builds. It's generally accepted that we will have 33 discipline as our pool to work with, and I'm hoping that we can extend that with items, but IDK for sure I haven't looked through the databases enough yet to see what people have found.
This is kinda hard to work with, because you need 10 for vault, and 21 for Preparation, if you use perfectionist that makes the cost 16.2 for Preparation, and 8 for Vault, which would allow you to use them both one after the other. With that choice for Preparation, I highly recommend Alabaster Rapid fire instead of Impale, since you'd get a slow that you really really need for this build past NM, but as you said it will adapt from here, but just pointing out a really good option for this build.
In terms of passives I feel like the DH's are the least appealing, many of them are so situational, that you'll never get as much overall use out of them as other classes have the option of, the best one if Cull the Weak, since we will be kiting a great deal. But I just really don't like half of them.