Forgive my ignorance of the rules as written, but just to cement these things in my head... 

1) You can challenge another player's introduction of a fact at any time -- no exceptions.
2) You can interrupt another player at any time, except when:
    a) that player is framing the scene and he has not yet declared an event as fact
    b) when the player is spending his/her bonus dice from the outcome of a complication

It looks like a great game btw!  I'll have more to say once we actually play, but so far it looks like something I can my friends interested in.

Thanks in advance...

I can't edit my previous post, and I realize I forgot to detail my other two questions mentioned in the subject... so please forgive this double-post:

3) A player can introduce a new Tenet as fact at anytime on their turn.
4) A player can introduce a new Rules Gimmick at anytime on their turn.

All yes, you've got it.

Look forward to hearing about your play.

Ralph