Yes, nesting is just fine and is often a good idea if you want to easily fade out something, as alpha is cumulative.
Using UIWidget on its own is also fine, as it's a blank invisible rectangle that is capable of receiving events if you put a collider on it.
UIWidget is not a new panel. Panel is what collects widgets and creates draw calls. UIWidget is just an invisible region that you can also use as a handy container. One example usage: making an interactable button that has a clickable region extending past its visible sprite.
