Always after foundation and concealer: their job is to set those creamy products so they last longer and don't move. Applying them beforehand would stop the foundation from blending well. The only partial exception is when someone uses coloured powder as their only makeup for the day, applied directly over moisturiser, for a quick, natural finish with very light coverage.
These are different categories that sometimes overlap. "Translucent" refers to colour: it's a powder with no pigment that sets makeup without altering the shade of your foundation. "Pressed" refers to the format: powder compressed into a compact, handy for touch-ups when you're out. In fact, there are translucent powders in pressed format and also in loose format. What matters is first deciding whether you want colour or not, and then choosing the format that best suits your routine.
It depends on when and how you use it. Loose powder has a finer milling, gives a lighter finish and is ideal for setting your makeup calmly in the morning; however, it's less convenient to carry around. Pressed powder offers slightly more coverage and fits in any bag, perfect for touching up shine during the day. The classic, very effective combination is to set your makeup at home with loose powder and touch up on the go with pressed powder.
It doesn't have to, but misuse can make dryness more noticeable. Powder absorbs some surface oil, which is desirable on combination and oily skin, but too much on dry skin can highlight dehydrated areas and fine lines. The key is to moisturise well before applying makeup, choose finely milled or luminous-finish powders, and apply them in a very light layer, only where truly needed.
Before adding more powder, remove excess oil by pressing the area with a tissue or blotting paper, without rubbing. Then apply a minimal amount of powder, ideally translucent or mattifying, pressing with the puff or a small brush only on the shiny area. If you add powder directly over built-up oil, the product will cake and create patches that are more noticeable than the shine itself.