Sorry, you asked about some specifics, and I didn't answer them.
Peer Gynt is excellent music, and is about as serious as music gets, in my opinion, yet it was written as incidental music for a stage play, so it is clearly an exception to the rule, as is Mendelsohn's Midsummer Night's Dream. The Sound of Mucus is not an exception, I do not think the music can be seperated from the stage.
I suppose one has to look at how the composer viewed the music. Did he compose the music to stand on its own, or did he compose it as a backdrop for the action? Symphonies are sometimes written with a story in mind, music for an imaginary play, if you will. In the end, however, the music stands alone, and that is what makes it 'serious' by the definition I was given.
It's not uncommon for a composer to write variations on his stage music designed to stand independent of the incident for which it was originally written. Overtures, entre-acts, and end credits are often composed using incidental themes from the play or movie. Some merely string together these themes, as a sort of musical rehash of the film, but others put together a musical work based on those themes that has a character of its own.
Ballet, IMHO, is serious music, using the dance, not so much to accentuate it, but to express it visually as well as audibly, sort of like music for the deaf.
Quod scripsi, scripsi.