As a diplomat, you're goal is social encounters would be to get the other person to do something for as little cost of you as possible. How would this be modeled?
In another person, there are two things of importance: the person's idea of you (your character) and their reluctance to do the thing you said to do. You need ti preserve and shine the former while reducing the latter. Nobody will do anything for you if they think you are an evil monster. They'll do anything if they think you are their savior. The better impression you have, the better reputation you have in a faction, the less reluctant they'll become. The more impossible the demand the more reluctance they'll have. Instead of attacks, you make types of argument (appeal to guilt, appeal to sentimentality, appeal to logic/reason, appear to character's intimidation levels, appeal to duty) and resources (bribe, offer lucrative position at a faction you have influence in, show items representing authority like permits, etc.). You can easily persuade a maid to reveal gossip from overhearing conversations of their master easily but it will be near impossible to convince her to kill their master. Thus a leveled combat-like encounter.
Weaknesses and strengths would depend on what motivates and demotivates a character. A simple example would be that a trader is motivated by greed and demotivated by fear. If you want a discount from him, you have to use arguments and social devices that appeals to his greed. "Invest" in his business and he'll give you a discount or information. Give him connections for his business and he'll add to your reputation. Try to terrify him and he'll do nothing for you once he gets away from him and hurt your reputation, even if you are able to take everything.
Character goal: most characters in D&D and RPG games usually have you as a character that are supreme masters of violence, able to kill Big Bad Evils while gathering power and wealth, sitting on top mountains of dead enemies or treasure. In this game, the goal should be to become someone of such status and influence that you can get anything done by a wave of your hand.
An idea for game-keeping: a stat representing the security of the character's position with their employers. It shows how much on-mission you are with their activities and if it drops to zero, the player has a social death and can no longer operate as an agent. There is no good/evil, just positive and negative reputation and this standing.
Guns are mechanical by their nature. They use the gas generated by the exploding cartridge to function, to cycle the next round. And also human mechanical strength. So crafting regular firearms guns would be a mechanical skill. If its a fancy Death Ray or Tesla gun or Gauss-gun or something similar, it would be Electronics skill.I'm considering removing gunsmith as it dosen't add much, but it probably represents something a player might focus on.)