One reason you are having difficulty getting an answer is that seal script is hard. The way characters are written on seals is not the usual writing. To make things worse, there are multiple possible ways to write the same character in seal script. So even native speakers who can read Chinese often have difficulty deciphering the characters on seals.
My impression is that this seal does not say mengchen, but you will want the opinion of someone with specific expertise on seal script. I don't know if there is someone with that type of expertise on the forum.