1) Wait, you can remove a chapter? I don't think you could accidently remove a page either.
2) There is no account of coauthors removing or sabotaging parts of storygames, and if you do not trust a coauthor, they shouldn't be your coauthor.
3) That is why people unpublish their game and make new paths on it but don't connect it to the story until it's finished and republish it.
4) Most of this could be solved easily by a different feature already asked for; a chance to go back and restore an earlier version of the story that was saved. Though that would be much more trouble than it's worth, especially in implementing it in the advanced editor section.