All powers using discarded cards choose from among those discarded during the game.
Yes, with the (only, I think) exception being the Statue of Liberty Wonder.
Yes, indeed Statue is an exception (although technically speaking its only interaction with the discard pile is adding a card to it):
Take the three cards of the current Age which were returned to the box during the game’s setup. Choose and construct one for free, place one in the discard pile, and your opponent constructs the third for free.
They could have been a bit more explicit here to exclude the 2 great temple cards put back into the box. But for me "the three cards of the current age" and the fact, that no scoring for more than 3 great temples is evident, is clear enough.
Btw. both promo wonders are excellent design wise and a must have (like everything else ;-))