It would be terrific if there was a Sigil-like program for mobi.
The problem is that a mobi book is a binary database file that must be built
from its html source. Epub is source that's zipped up. A binary mobi file editor would be a fairly large undertaking (especially since there are still a few mysteries being slowly revealed about the proprietary format). It's much easier to convert... fix... and reconvert than it would be to code a wysiwyg binary mobi editor. And frankly... the end result wouldn't be any (OK... much
) different anyway.