Well yes it's quite different. Shooting and Stabbing people is a very open activity. If I pulled my gun on someone they might back down, or they might pull their gun. There is still a ton of roleplaying to be had.
Pick pocketing isn't that. You just do it and move on. Even if you use the peg challenge idea, or you have to draw a chalk cross on someone it's still something that happens. A GM has to come in and intervene and tell you that your item is gone. Now you know your item is gone but you don't know. It changes the way you play everything.
I have no problem with story/plot crew picking my pocket and then letting em know when I go for something it is gone. I'm happy for that to pretty much happen. But it feels uncomfortable when a player has the ability to run around picking everyone's pocket. Yeah I get that people do it to survive, it's a lowish resource game after all, but it has no limits.
And if they challenge is too difficult they caught so they don't do it. If they do get caught they probably aren't long for this world in a game like this.
Yes theoretically you could have a character that goes around stabbing and killing everyone but they would suck. Conflict is good, outright antagonism not so much.
Ultimately Rock Paper Scissors is a pretty poor mechanic for anything. Better to just roleplay it. I know some players might be better at diplomacy than others, but that's pretty much the same as combat. I wouldn't want to be on the opposite team to Nick Pitt for example.
Between Engineering, Survival, Tech and Repair I think tinkering is probably covered.
Re: Pick Pocketing again
Not as a skill but as a player goal for a session this could work. I.e. Player X has been forced to work for someone. They want player X to get a bunch of money by end of session. One way is to pickpocket people. The challenge would be to peg 10-15 people over the night. Player X is really just looking for currency, so it's probably no major loss to any of the players. It is difficult goal, though each peg could be worth 5% of the goal or something and maybe there are a few other ways to make some coin.