Can you clarify on this, or perhaps show what your database and category settings are? I have a database that posts a topic to a forum, and a category within that database that overrides that and does not post to a forum, and I cannot reproduce a topic being created anyway.
Regarding the canonical tag - this is by design, currently. The canonical tag is generated dynamically based on the content the user is viewing, and when this is generated the system is unaware that a topic could be linked to a record.