Ok, First of all I really am not sure about the 2nd question so I will leave that for someone else to reply.
Now as for the first question I can only explain it the way I understood it. I believe that the silhouette you see is Regan. Yes she is restrained but I have the opinion that she could free herself at anytime. Just like her opening the dresser drawer she did it once then Fr. Karras asked her to do it again she then says, “That’s much to vulgar a display of power.” So in a sense she is saying I can do what I want whenever I want and there is a purpose for everything I do. Now am I reaching? Maybe?
Also you could read it as like an apparition type thing. Or and maybe the most sensible version would be that it is Sharon up in her room taking care of her. But I like my first version the best. It gives more of a sense of mystery and it shows a higher level of the unknown power of the demon to the story line. Well, I hope I have helped! I may not know just what the screenwriter intended I can only tell you how I see it.
Later!
Erica