With regards to Mayhem, once the card is discarded, you can cast it at any point during your turn
The honest answer: it depends on the card
If you have better dual lands (anything from Polluted Delta to Watery Grave to Drowned Catacomb to Clearwater Pathway), adding them to the deck over the current duals (starting with Temple of Deceit and Choked Estuary) will improve the deck, although I probably wouldn't run out and spend hundreds of dollars on lands just to upgrade Dimir Thieves
The card plays rather like a God-Pharaoh's Gift that uses all graveyards, not just yours, and I expect it to be good in the same decks
With the update to the game, there is also a digital version you can play with online right now
The card is currently selling (with preorders) for $35-$40, and it's one of the most exciting finds in the set