@abs-oluteM I found the book reviews of the novel from an online bookstore last time before the drama started. Majority of the readers loved the original novel but it was a tearjerker and it was sad.
One Hottest perhaps read the novel but so far, her feedback on the adaptation is that they changed some parts, but no question was arise yet about the "crown princess".
Book review page
And also, I figured out the meanings of the two bright posters released for this drama
I also watched the long highlight teaser again but this time with subtitles, and it seems that the main plot of the drama is more on the establishment of the loyalty by Deok Im for Yi San. I think Yi San will encounter more hardships later on because people question his worth as a successor, and this is where Deok Im comes in.
For someone like me who is never fond of saeguk, I just noticed today while checking photos and teasers that Deok Im's uniform color definitely changed later on, especially when it was shown that she is doing her storytelling for Yi San. She eventually will get promoted then.