I'm going to use the terminology I like:
- PC--character under control of a player
- NPC--character under control of the GM
- DPC--character "owned" and "directed" by the GM but played/portrayed by another player/co-GM. q.v. "monster" in LARP.
It's all semantics, really. If it suits you better to call DPCs PCs with no char sheet, or to argue that we're all just a bunch of C's (ain't that the truth), that's fine. We applied that distinction for years as a way of getting marginal players hooked on playing, to introduce experienced players to GMing (as a co-GM), etc.
Much of the argument above has been assuming all players are at the same experience level, and will be equally happy and confident to integrate into a new game. That will never be the case. The DPC is just one trick to draw people in. As people get more confident all of these distinctions merge--which is why my (insular, reactionary) extended group just doesn't talk about this sort of thing any more.
I believe one of the primary functions of an RPG system is in fact to give people 'permission to be someone important' as you say. And while I have seen the PC/NPC difference you describe, I've also seen the same with self created characters, and I believe the root has more to do with emotional distance that specifically NPCdom (though the idea that an NPC gives a player MORE permission to be important than a PC is still an odd one to me)
In what way? What psychological factors and/or system mechanic is behind that agency?
Barriers to perform that are based on how well the new player knows the existing group; their willingness to integrate with the group; their confidence in playing in front of a new group they don't know.
"Importance" is the wrong word. A better definition is need to establish the character. A walk-on DPC doesn't have to compete with the shared history of the rest of the group.
What about the opposite, where the NPCs player wants to continue playing that NPC and join the group?
That's rarely a problem in-game. I have done it. It's only ever been a problem out of game when a player expects to be invited back and aren't (usually they're upset because of crossed wires).
Personally, I don't believe in PCs or NPCs, just Cs which change function depending on context.
I tend to agree, but defining roles and expectations is useful for a lot of people.