You tell them at the time you trade the exchange order for the pass when you want it to start, and they write it in. This is true for the regional passes as well.

For 4/11 to 4/17, it is not really high season so advance reservations (like before you get to Japan) are probably not necessary. Using the eki-net website (which requires that you pick up the tickets within the JR East region only) has a number of problems. One of them is that if you do anything the slightest bit wrong or miss your train, your credit card will be irrevocably billed for the ticket. As a result, most of us who have gotten JR passes have tended to consider this website, if at all, only for REAL high season travel, such as Golden Week.

It's perfectly OK to buy multiple JR pass exchange orders in advance. In fact, if you don't use them, most of them seem to allow you to return them when you get home for only a 10% charge.
Won't save you much time since you still have to trade the exchange order for an actual pass when you get there. The folks at the JR West desk at KIX seemed to me to be the most efficient exchange agents of all the ones I have dealt with.