A flaw in the orginal and all alternative systems as I see it is that every party should from a roleplaying perspective leave the guys that are bad in a skill out, unless the weak characters are loose canons. Metagame people might join even if they are very bad in social skills just for XP.
What is your thought about this? In combat everybody contributes, because everyone is good in combat and even if you are not (badly designed PC) you aren't hurting the party; maybe on healing if you have low AC and NADs and do very little damage. But you see my point?
Your system works good in combat. The guy/guys that are good in thievery would disable the trap, while the rest would fight, and everybody should share the XP. The problem is pure skill challenges.
In physical encounters everybody might contribute. Say you are escaping a dungeon or something. You are required to contribute or else you are caught. So that one is ok... I am thinking more about social and knowledge...
In social I have thought about what if the skill challenge goes both ways? That the NPC/NPC's makes contact with say a guy that says nothing... That is both realistic and would include more people, and also might lead to a fail if the main talking character is very good in say diplomacy (cha 20@lvl 1, trained, +2 racial, +2 background, +3 skill focus).
The problem is knowledge challenges though. I don't see how you could include the inactive party members here at a