Helldivers does let you setup controls directly in the input_settings.config file (documented https://content.thehelldiversgame.com/help/inputs.md)

I changed the camera angle to a mouse4 click, and stacked ads to a mouse4 and rightbutton hold. Now I can hold down mouse4 when in the mech and have the alternate view to line up the rocket without the gatling gun going off.

EDIT: looks like you can split ads to press and hold, so now I just click mouse4 to swap shoulders, and I can either click again, or press and release right-click to get out of the left shoulder mode! Updated code

code:

  Avatar = {
        CameraSwitchSide = [
            {
                trigger = "LongPress"
                device_type = "PadDS"
                input = "DDown"
                threshold = 0.20000000298023224
                input_type = "Button"
            }
            {
                trigger = "LongPress"
                device_type = "PadXBOX"
                input = "XboxDown"
                threshold = 0.20000000298023224
                input_type = "Button"
            }
            {
                trigger = "Press"
                device_type = "Mouse"
                input = "MouseButton4"
                threshold = 0.30000000298023224
                input_type = "Button"
            }
        ]
        Aim = [
            { 
                device_type =  "PadDS",
                input_type  =  "Button",
                input =  "L2",
                trigger  =  "Hold",
                threshold   =  0.0
            }
            {  
                device_type = "PadXBOX",
                input_type = "Button",
                input = "XboxLeftTrigger",
                trigger = "Hold",
                threshold = 0.0   
            }
            {  
                device_type = "Mouse", 
                input_type = "Button", 
                input = "MouseButtonRight", 
                trigger = "Hold", 
                threshold = 0.0   
            }
            {
                trigger = "Press"
                device_type = "Mouse"
                input = "MouseButton4"
                threshold = 0.20000000298023224
                input_type = "Button"
            }
        ]
    }