If I buy a Disney Club membership will it give me discounts to WL? And if so, would I be able to combine that discount with a Disney code if they happen to come out with one?
Memberships are no longer available as they are phasing out the Disney Club at the end of this year.

No matter what the discount, I've never found any that could be combined.
Can I make reservations through DU without a code and then call back to apply a code if one comes out?
While I've never used DU, it's my understanding that they will keep an eye on the codes and get any available ones that come out applied. I'm sure there are others who are more familiar with how this works. If you book yourself, you can make a reservation at any time, then if a code comes out you can call and see if there is availabilty for your resort and room choice and have the discount applied to your current reservation.
If I call and make reservations now will I need to pay for the room in full before I get there, and does it need to paid in one lump sum?
It depends on who you book with. If you make the reservations yourself, you will use either Walt Disney Travel Company or Central Reservations. Each have their own rules regarding deposits and when payment is due. I've heard that Disney is changing their reservation policies somewhat, so it's hard to know exactly what you'll be told. People have reported being told different things. The general rule is that CRO is more liberal with deposits, cancellations and payments of the balance. Neither would require you to pay the entire balance in one lump sum up front though.
As far as crowds are concerned, if you are staying before the week of Christmas, crowds are generally very light. Honestly, I wouldn't encourage a first Disney trip for the week of Christmas! The crowds are extremely heavy from about the 21st right through New Years. It really is amazing to see the difference from one day to the next. I'm sure you'll love WL. We stayed there from 12/19 - 1/01/03 and I don't think there's a better resort at Christmas, it's really beautiful!
It can be overwhelming when you first start learning all this, but by the time your vacation arrives, you'll feel like a Disney expert! Happy planning!