This is an issue I've been working through for some time. I still don't have an answer other than to say that the scenarios you describe don't necessarily want to be attached to the triple.

Others made useful comments to my post here: http://www.dynamicorange.com/blog/archives/semantic-web/reification_tri.html

Some of the things you want to do can be addressed using named graphs (provenance being one) and others by reification (confidence, for example).

rob