A: Yes, Double and Triple Agenda are considered mutually exclusive, since they’re both determinants of the same parameter: the number of simultaneous Goals allowed. There’s only one deck that contains both* so we didn’t think to specify it, and left it with the “filler” small print about taking immediate effect.
Note that any Goals in effect with Double Agenda stay in play if it is replaced by Triple agenda. Triple Agenda simply “opens up an additional slot” for another Goal. Of course, when reducing from Triple Agenda to Double Agenda, the active player would choose which Goal will be tossed, leaving the others in play, just as when going from Double Agenda back to the regular single Goal in play.
* Holiday Fluxx, though this issue could also come up if you’re playing with the combined Star Trek TOS & TNG double-deck or some other combo of combined decks.