1. There are no examples of drag and drop inside an iframe there.

2. This does not work

handle = sb.cdp.get_nested_element(iframe, hadle_selector)
position = handle.get_position()
x1, y1 = position.x, position.y
x2, y2 = x1 + 50, y1,
sb.cdp.gui_drag_drop_points(x1, y1, x2, y2)

3. What if the element doesn't exist in the destination?
   How do I move the starting point by x and y pixels relative to the destination?

You'll have to use sb.cdp.gui_drag_drop_points(x1, y1, x2, y2) and determine the coordinates yourself.