The only time you MUST show both is only if there is a showdown.
You MIGHT have to show both( or the second card) , if for instance you bet, and no one calls, you can show one card. BUT, if someone asks to see the other card you (or the dealer) are obligated to show it. The dealer will never, (or should never) expose the second card if no one asks.
So he shows a 2 of diamonds, that just helps you narrow what his other card is that much more...oh well.
Yes, but if he shows one, being able to ask to see the other one REALLY helps narrow down the second card : )