Is there any way to invert controls? (negative walk speed)

You can do this pretty easily. All you need to do is copy the default ControlScript and add a single line to the MasterControl module.

Getting the Default ControlScript

This is most easily done by going into PlayMode in studio and looking under Players > Player1 > PlayerScripts in the explorer. You should see two local scripts, the one we want being called ControlScript.

Modifying the MasterControl Module

Now that you've got the script, copy it to StarterPlayer > StarterPlayerScripts. Open up the 'MasterControl' module parented to the script and go to line 97 (should be an AddToPlayerMovement function). Change this function to the following:

function MasterControl:AddToPlayerMovement(playerMoveVector)
    playerMoveVector = -playerMoveVector
    moveValue = Vector3.new(moveValue.X + playerMoveVector.X, moveValue.Y + playerMoveVector.Y, moveValue.Z + playerMoveVector.Z)
end

The 'playerMoveVector' is now reversed so left is right, forwards is backwards, etc.

If you wanted to be able to toggle this mid game, I would recommend using a RemoteEvent or BindableEvent.