1. Players satisfying the conditions may choose to enter into a pact - its not automatic?
It is automatic.
2. When do you enter into a pact? At the start of each round - just before everyone bids?
At the end of the turn, when everybody places their pawn.
3. Do you still bid when you've entered into a pact? If you do, and you turn over a devil do you pay 50 to everyone else who entered into a pact for this round?
You do. And as long as you have enough money, you have to pay in case of turning over a devil. If you don't have enough money, the bank pays for you.
4. From #2 & #3, the rules say that players entering into a pact must "lay open their assets for all others to see". If you reveal your betting chips then make a bid, I presume your revealed chips don't include the chips in your hand that you're about to bid?
Correct. Best way to make the bid secret (before revealing) is to take as many chips as you wish in one hand, sort from these the ones you want to bid into the other hand, then reveal these simultaneously wit the other players and lay the remains back to your assets.