Actually, in terms of scripting, Houen Mirror World is more of a hybrid scriptfic-narrative , as Houen Mirror World still has narratives. I did try to read Houen Mirror World, but I'm terribly picky on scripting formats because I just can't stand a scriptfic without stage directions >>; If you must insist on judging Houen Mirror World's format, it's either classifying it a hybrid, or call it a 6/10 for format correctness... missing out on the stage direction is a terror for a scriptfic, and it just instantly loses a lot of its meanings...
Script-fanfics are no way forced to humor, and in fact most professional script-novels have very serious themes. Miller's "The Crusable" done almost entirely in script besides the background context talks of religious fear, which is quite a controversial and serious topic (along with sexual immorality, manipulation, and many other themes). It's a script, and it's serious.
When a script fanfic is written nicely enough (almost impossible in a fanfic skill level of writing though), it conveys much more than what a 3rd person narrative can do.
1st person excel in first hand feelings and giving readers a sense of closeness. Script convey to us a theme/themes as we analyze the action of a character and how this theme affects whatever they do. 3rd person is the mixture in between. Narratives are more personal, while script always look on an issue as a whole, but both can talk of character emotions just as well.