If you have generated a ToC.ncx and/or inline (HTML) ToC already, you do NOT want calibre to create one during conversion. That is meant only for ebooks which do not yet have a ToC.
Under "___ Output", you will have an option to enable or disable generation of an inline ToC. Note that the tooltip says it is only useful (or wanted, really) if the book does not aready have an inline ToC.
As for closing threads -- no, MobileRead policy is to leave them open. Only moderators can close threads (as a last resort).
You can't put [solved] in the title either -- which I know some places allow and suggest, but again, here we don't.