Using Karabiner for multi key to multi key remapping
Ok I solved my two specific problems, though I am still not sure how I would replicate multiple key presses if more than one of them were not modifier keys.

Put the following two json files in user/.config/karabiner/assets/complex_modifications

{
  "title": "Shift + Enter",
  "rules": [
    {
      "description": "Shift + Enter to Option Enter (newline in messenger)",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "return_or_enter",
            "modifiers": {
              "mandatory": [
                "shift"
              ],
              "optional": [
                "caps_lock",
                "option"
              ]
            }
          },
          "to": [
            {
              "key_code": "return_or_enter",
              "modifiers": [
                "option"
            ]
            }
          ]
        }
      ]
    }
  ]
}


This one (shift enter) is really frustrating me but not because of karabiner. In messenger i need to option+enter to get a new line character, but in the browser option-enter refuses to do anything. I wish I could get a key combination that does a consistent "safe" new line character, rather than making me fear I am going to accidentally press enter to send or complete or publish or something like that.

{
  "title": "cmd + angle brackets for workspaces",
  "rules": [
    {
      "description": "Use CMD + comma  (left angle bracket key) to switch to  left workspace",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "comma",
            "modifiers": {
              "mandatory": [
                "command"
              ],
              "optional": [
                "caps_lock",
                "option",
                "control"
              ]
            }
          },
          "to": [
            {
              "key_code": "left_arrow",
              "modifiers": [
                "control"
            ]
            }
          ]
        }
      ]
    },
    {
      "description": "Use CMD + .  (right angle bracket key) to switch to  left workspace",
      "manipulators": [
        {
          "type": "basic",
          "from": {
            "key_code": "period",
            "modifiers": {
              "mandatory": [
                "command"
              ],
              "optional": [
                "caps_lock",
                "option",
                "control"
              ]
            }
          },
          "to": [
            {
              "key_code": "right_arrow",
              "modifiers": [
                "control"
            ]
            }
          ]
        }
      ]
    }
  ]
}


A solution to a similar question can be found here: How to bind a key to press two separate keys