I know this is late and you probably have found a solution that works for you, but if not here is only the relevant part of the camera movement which is contained in the player script:

var viewport : Viewport
var viewport_center : Vector2
var target_direction : Vector2
onready var camera := get_node("Offset/Camera")
func _ready():
     viewport = get_viewport()
     viewport_center = viewport.get_visible_rect().size/2
func _process(delta):
    # get direction from center of the screen to current mouse position
    var direction = viewport.get_mouse_position() - viewport_center
    # remap value range to 0-1
    direction = direction/viewport.get_visible_rect().size.x
    # lerp to add delay and bounds to camera movement
    target_direction = lerp(target_direction, Vector2(direction.x, clamp(direction.y, -1, 0)), delta*10)
    # movement inversions transfrom in local space
    camera.transform.origin.x = target_direction.x
    camera.transform.origin.y = -target_direction.y

You basically take the center point of the screen and lerp the camera position to current mouse position with a clamp that defines your bounds. The relative camera position must be at (0,0,0) for it to work.

Hope that helps,

Cyanine