Yuuki is one of the characters who changed the most from web novel to light novel and this is the arc where the story itself has drastic changes from the web novel, so you should not think of web novel as a source material because it isn't.
Yuuki and his secretary has been in the manga since Rimuru went to save the children, the only thing is that it was revealed that she is Demon Lord Kazaream(a dude in a cute girl homunculus body)
No, the mangaka is not being disrespectful to the source material. The only change from the LN is that they didn't reveal Yuuki's identity as a villain in this scene, but it is more impactful this way, otherwise you would get 40 pages of Yuuki's feet being shown.
It was also mentioned that Rimuru was in his human form when the Farmus' king saw him.