Sorry, but there’s no way the sights are off in 3rd person because when you look through the lens you’re in 1st person. I guess what you’re referring to is the crosshair in 3rd person, the crosshair in 3rd person isn’t very exact, that’s true.
About gun customization, if you try to customize the gun in 3rd person, you won’t see the gun up close and the character’s body will occlude your view…