On your first question, those are rating limits.
Look at your coffeemaker, "capacity 10 cups" etc. Can you make 4 cups? Of course. They are saying if you put more than 10 cups, it won't work. Your S-rated (112mph) car tires are also safe at lower speeds. Etc.
125V is the max limit for the socket (due to insulation strength). You don't get to choose the voltage in your house, the power company supplies 120V in North America. This would be the wrong socket, say, for Europe where line voltage is 230V.
75W is the max limit for the socket (due to thermal heat from the bulb). Since the ruling factor is heat not light, you are allowed to use any bulb less than 75 watts actual. So if you have a 26-watt LED (bright like a 150W incandescent), that is fine since it's well under 75W.
On your second question, if your breaker is 20A, then use of 12 AWG cable is mandatory all the way from the panel to the ceiling box where the chandelier attaches. This is another "rating-like" thing; you're perfectly welcome to use larger 10 AWG cable but you cannot use the smaller 14 AWG.