I will try to answer your questions.
When on a beam reach, your body is close to the mast and mast base and the boom height is not so critical, at least for comfort. Heading upwind, you still want it high (shoulder/neck/chin) range to keep the sail more upright.
When running downwind, your body extends over the back/side of the board (as opposed to the side) and is much further from the mast base, which causes the rig to lean toward the back of the board, which results in the boom being much closer to the deck of the board. If the boom is too low, you will not be able to keep your front leg straight and you will be in somewhat of a squat position. The front leg should be straight and the back leg is either straight or bent depending on how much chop you are in. A few times, my boom has slipped down my mast while on the water 10cm or so. It's almost impossible to head deep downwind with a low boom. Try it sometime at low chest level, then at neck level and you will see what I am talking about.
Downwind, the rig does not lean forward, otherwise you would get tossed over the nose. I am talking about planing with speed. Most of the weight is still on the back leg/foot
Upwind, both legs tend to be straight, with most of the weight (90%+) on the back foot. When cresting chop, legs should bend, absorbing the bump in order to keep the board on the surface of the water.
Hope this helps.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.