It is not ALWAYS, it is sometimes. Other doujinshi artists do it too. Lead pencil backgrounds will smear and will distract readers from the foreground, which is the focus. I'm against using THEIR profiles because, as I stated many times with references to other artists, it is a style choice.

This is a common thing in manga/anime, depending on artist or style or the manga or animation. The mouth rides up the cheek so they don't have to alter the face structure- either as a style choice or a shortcut. Here are some random samples.

Depends on the artist.

Also, what cheek mouths? Check again.

Characters and landscapes. I started sketching landscapes. Perhaps try there.

From Hatsune Miku, a famous pop idol character from Japan. Lots of people love her.

I think so.