As a general rule, for close range, use a high guard, and for long range, use a low guard. In close, hands are deadly so you need a high guard. In addition, in close, kicks are difficult to see coming and they are fired from a high chamber so a high guard protects the head. At long range, hands cannot reach you without the opponent closing the range so a high guard is not as crucial. At a long range, kicks are fired more from the floor than from a high chamber, so the kicks are rising from the floor and are easily stopped with a low guard.
As with everything else, the guard you should use is the one that works best for you under the circumstances. The guard that works best for me is a high, extended, open-handed guard. With the arms extended rather than close in, I can pick off attacks easier, and the hands are closer to their targets.
Q2: I am very close combat oriented (hockey player) and I throw knees and fists a lot. Do I get points when I punch the dot in a match?
What constitutes point depends on the rules of the competition. Your instructor will be able to tell you what is a permissible scoring area and scoring technique under your completion rules. I too am a close in fighter. Close in fighters like to use counterattacks. Learn to block and then attack, such as blocking with an arm and then letting the arm continue into an attack, and learn to simultaneously block and attack, such as blocking with one arm while the other arm simultaneously attacks. When the opponent initiates an attack, you immediately close in with an attack. I will chance getting hit with one point when I rush in if I can get two or more points in my counterattack. Use legs for blocking. From a high guard, when a kick fires, lift your forward knee to your elbow. Then the arm is blocking high, the leg is blocking low, and both the arm and leg are chambered for a counterattack. Opponents find that kicking your shin hurts them more than you.