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1. The diamonds are blocked, preventing you from collecting five tricks in the suit. However, it would be wrong to take a spade finesse at this point, because if it lost to the king in the North hand, you could go down one, losing four hearts and a spade. A far better plan is to lead a heart from dummy at trick five and discard a diamond! This allows you to unblock the diamonds so you can eventually score five diamond tricks.
The first two tricks revealed that North started with at most five hearts, so there is no way you can lose more than four tricks on the suggested line of play.
2. There is a possibility of losing an extra trump trick on top of the obvious two spade losers and the trump ace. Your best chance to make the contract lies in winning the spade queen with the ace and returning a low trump from dummy.
Let's assume South follows low and that you win with the king, which is very likely in view of the bidding. The correct play now is the queen of trump. If the trumps are divided 3-2, all your worries are over. South can take the ace of trump and cash two spade tricks, but you will then have the rest of the tricks regardless of what South does next.
The key play is the queen of trump at trick three, rather than leading a low trump to dummy's jack. Leading the queen guards against South having started with something like [S]KJ1083 [H]A8 [D]92 [C]K854, in which case he would take dummy's jack of trump with the ace, cash two spade tricks and then lead another spade, promoting North's ten of hearts into the setting trick. Preserving the jack in dummy to overruff the 10 if need be, protects against this possibility.
Tomorrow: The power of a long suit.