Though I'm not very familiar with it, kristensboard.com allows snuff and underaged subjects. I'm sure the rest would be fine too.
And ASSTR seems to have had issues for quite a while, now.
ASSTR has been having server problems that is causing site outages and other issues. The site admins are working on the problem and are considering moving to a cloud based system. I'll post the latest announcement from ASSTR in /dis/
I'd love to know if there is a place to post stories containing torture and snuff with underage characters, that also has large amounts of sexual content that might not really be entirely guro. I suppose the answer is to post here as there is already a lot of that in /lit/
If people/site admin don't like too much rape and other forms of sexual depravity that are not guro then the worst that can happen is that the story won't get a favourable response, or might get removed by the site admin. As far as I'm aware there are no rules on GUROchan regarding story content.
I have a webhost where I got around 19.8MB left, I dont know if storys involving children is legal where I live tho but the server is increadible stable.
If you can get on IRC(join #r9k, im banned in #guro) we can figure something out as you proabably need to send me tthe storys
stories.xnxx.com doesn't appear to have any content restrictions at all. I also can't find a way to change the background color.
bdsmlibrary.com is a much better organized site and fits most of your criteria(free, accepts rape/snuff/necrophilia, has a "all characters are 18 or over" rule that it doesn't seem to enforce), but the admins have been MIA since April and nobody knows if it'll ever be updated. Still, it's someplace to keep in mind.