Emile de Maat
1. Can a familiar be used to explore & map out a new section of the dungeon?
No. Familiars cannot open doors, so they cannot reveal an unrevealed section of the dungeon. See also this thread.
2. I'm a bit confused about "exhausting" a card. Is it in play continuously?
Yes, they are in play continuously. Cards that "exhaust" can only be used once per round. To indicate you've used it, you turn it sideways. At the start of your next turn, you refresh the card, turning it upright again, indicating that it can be used once more.
Some cards have both exhaust actions and a continuous effect. The continuous effect stays active even when the card is exhausted, as per the FAQ:
Descent: Journeys in the Dark Base Game FAQ wrote:
Q: If an item such as a shield is exhausted, are its passive abilities (if any) disabled?
A: No. The only limitation is that the item cannot use any ability that requires it to exhaust until it has been refreshed.
3. Finally for treasure, once chest is open-ALL players recieve 100 coins?
Yes; all heroes get the coins from chests and money markers.