What's the problem with this Karabiner complex modification? I added the following rule to .config/karabiner/karabiner.json by importing a similar rule and changing the previous key code to "option". But the rule has no effect. Is there a problem with this rule?

"complex_modifications": {
    "parameters": {
        "basic.simultaneous_threshold_milliseconds": 50,
        "basic.to_delayed_action_delay_milliseconds": 500,
        "basic.to_if_alone_timeout_milliseconds": 1000,
        "basic.to_if_held_down_threshold_milliseconds": 500,
        "mouse_motion_to_scroll.speed": 100
    },
    "rules": [
        {
            "description": "Simultaneously press Option + Del to get a ForwardDel",
            "manipulators": [
                {
                    "from": {
                        "simultaneous": [
                            {
                                "key_code": "option"
                            },
                            {
                                "key_code": "delete_or_backspace"
                            }
                        ]
                    },
                    "to": [
                        {
                            "key_code": "delete_forward"
                        }
                    ],
                    "type": "basic"
                }
            ]
        }
    ]
},
[...]


The rule is supposed to delete "c" in "ab|cd" if "|" is the cursor and upon pressing [Option] + [Backspace].

I used a simple rule for turning [Caps Lock] into [Option]. This rule works.

Also - is there a simpler method for reloading a changed karabiner.json than closing Karabiner-Elements and starting again?
option should be defined as a modifier key. simultaneous is used for non-modifier keys. Besides, you'd better separate your mappings into a file from .config/karabiner/karabiner.json, 

~/.config/karabiner/assets/complex_modifications/delete-forward.json

{
  "title": "Delete Forward",
  "rules": [
    {
      "description": "Simultaneously press Option + Del to get a ForwardDel",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "delete_or_backspace",
            "modifiers": {
              "mandatory":[
                "option"
              ]
            }
          },
          "to": [
            {
              "key_code": "delete_forward"
            }
          ]
        }
      ]
    }
  ]
}


Go to Karabiner-Elements Preferences -> Complex Modifications -> Rules, Click "Add rule". Then you'll see the new created rule from this separate file.