In practice, if I want all the players to engage a complex challenge, I try to make sure that I establish the fictional premise in such a way that all have something to do. For example, the party's last complex social interaction was with some witches, and the dwarf's contribution to that skill challenge was his showing as a pit fighter against some spiders living underneath the witches' house.
But in a less complex challenge (like the 6/3 challenges described in my OP) I'm a bit more relaxed, working on the theory that even if one player can't think of a way to do anything useful, and that PC is just standing there getting eaten by a bear or generating failures, it won't make much difference. The other PCs will be able to pick up the slack.
Well, I can't say I dislike your approach and it has indeed prompted me to break out the dusty 4e books again and review some of the rules. I only spent a few months with it right after release. I just could never get into it as a game.
However, in any system, you can make something of it if you have the talent. I'm still not convinced that the game needs a structured out-of-combat encounter ruleset. Most of the examples I have seen in this thread and others involve talented DMs turning the proverbial water into wine IMO.
The process you describe just seems too...mechanical for my tastes. I mean, in a very loose sense it does describe pretty much what I already do for an out of combat encounter. I decide on the fly how skills play into it, what will change with the story as things evolve and what each player can bring to the table. For whatever reason reducing it to a structured challenge system such as you describe feels like a step backward to me.
I run mostly "by the seat of my pants" going on the theory that I have no clue what a player is going to do. Most outside of combat encounters evolve fairly organically in my game - in fact I can't usually tell you beforehand where an outside of combat encounter may evolve or with what NPC/situation. Setting up a bunch of skill challenges following some sort of structured ruleset beforehand is not at all feasible.
I can see how the system might be invaluable to a novice DM (and I'm not saying your approach is that of a novice, it looks extremely well developed and skilled) so that they can have a laundry list of how everyone will be involved with every encounter of a pre-defined module; combat and otherwise. Which doesn't surprise me too much because I really got the feeling that the 4e DMG was written more as a training manual than a reference book. Altogether that's not a bad thing, we need more competent DMs in the hobby and frankly, unless they replace the DM with a computer as some suggest, it's the only way the hobby will grow.
My concern is that this sort of "rules for everything" thinking encourages changing the game from a RolePG to more of a board game by quantifying every aspect of it. So the novice DM that picks this up, may just never truly get that shift in thinking. I already saw that a bit with 3e where suddenly since there was a very defined rule for nearly everything you could do, DMs and Players had to be overly cautious of allowing anything outside the scope of those rules upon rules lest their new ruling cause some sort of rift in time-space
At any rate, I do plan to give the skill challenge idea some more thought as a result of this thread, but for the moment I prefer to keep the mechanics a little more fluid in my fiction. I appreaciate your enlightening me on the creative and intelligent sides of this system!