dogzy, In your first situation I think that the player was wrong to not show both cards, but IMO, if he wanted to throw his other card into the muck after seeing that the opposing player had him beat, let him. The proper procedure is for the dealer to ask to see the hand of the SB, which he did. A good dealer should ask to see the complete hand so he can properly read the hand. You stated that the other player exposed his hand (prematurely), obviously before the SB showed his complete hand. Players at the table have a right to see a called hand, but it is not the responsibility of the dealer to turn the card over. What if, by turning the card, it revealed a winning hand? Players have a responsibility to protect their own hand. If they are in violation of a rule then they should be given a warning, but I am not in agreement to insist that a player turn over both cards if he decides to muck his hand.
In the second situation, I think that both hands (UTG+2 and UTG+3) should have been killed. I base this decision on the fact that two players have acted (UTG and UTG+1). This could be a tough one to sort out. I know that there are members that have a tough time defining what constitutes substantial action. If UTG and UTG+1 had stayed in the hand by calling or raising, the action would have continued, correct? Therefore, there should be no difference. I think the dealer should not have declared a mis-deal (that should not be his call anyway), the dealer should have asked for a ruling from the floor. It is my belief that the correct call is for the hand to continue.