The ability to imitate other people if you've heard them speak for a minute (Actor), the ability to forge notes and documentations if you've seen examples of such (False Identity), and a completely seperate identity (False Identity), all backed up by advantage on your Deception and Persausion checks (Actor)?
Throw in some form of shapechange (and maybe scrying) spell and you can cause anarchy.
Tried it once. Was a first level rogue Changeling with the Actor feat (the DM having granted us all a free feat). With expertise in Deception and Persausion I was able to successfully imitate almost anyone (unless the dice hated me). Acquiring official papers was the goal of the day, and when I achieved that... oh boy.
It does require a DM that interacts equally with the socialisation and combat pillars though.
Okay, I see.
Well, in that case I don't think I've missed anything - I had a hunch this was how the feat would play out.
Mechanically, I'm thankful the feat at least isn't trading in absolutes ("You automatically mimic anyone you study for 1 minute with no risk of detection").
Other than that, I'm afraid I can't give you a satisfactory answer to the greater issue. D&D has always been a game that severely undervalues what you can accomplish outside of combat. Just look at how long Charm used to work in previous edition...
So I'm afraid "fixing" irritating players is out of scope of any feat rebalance (and certainly for this review). Social problems is best fixed using social solutions. What I mean by that is: when we role-play, we generally resolve combat almost entirely by mechanics (rather than the player and DM duking it out
), and so combat imbalance needs to be fixed with rules.
But social issues are much more often (and in most but not all groups) handled at least in part by evaluating the player's performance (rather than the character's alone).
So if a player "abuses" his share of the spotlight, and derails adventures by influencing NPCs away from the plot, that is something I'd solve by having a talk out-of-game with the player. No rule can fix this; and certainly not a tweak to this feat.
More generally, I'd hesitate to call this player a "social power gamer" even though that is exactly what he is. I mean, when a minmaxer breaks combat, I respond by nerfing the rules elements that aren't proofed against that level of optimization. But since the purpose of the game is at least in part to become good at killing monsters, I don't consider that gamer someone that ruins the game.
But when it comes to social, I do. The game simply doesn't have any rules knobs and levels to control him with, in the way the rules supposedly allow power gamers to play without the risk of breaking something, so the only solution IMO is to play with people that voluntarily doesn't wreck campaigns using features like Charm or Actor.
Since most people don't find it interesting to minmax something that can be trivially broken, this isn't an issue for most groups, I believe. Most people simply go "sure I could spend our session making sure we solve the problem without actually going on an adventure... but why would I want to anything like that?".
Zapp
PS. Edited in a short mention in the Actor review that briefly mentions what you and I have discussed here, Yunru. DS 
