I've never had a problem with paraffin wax. Where can you get duck wax? What exactly -is- duck wax?
I've heard varying ages for processing ducks. What it boils down to is what the feathers look like under the wing. Do you see a bunch of pinfeathers? Wait a week. Are the feathers grown? Have at.
I've never heard of butchering after 12 weeks, and personally wouldn't bother unless the birds were going through a molt. Adults are a complete hassle to dress out.
With that being said, I'm talking about fast growing Pekins. Obviously slower growing birds would be fine to dress out at a later time, though not much later.
As for the hassle - don't make it into one. Get someone to help you and make a double-task of the job. Collect as much down as you possibly can to use in quilts, pillows, coats, and everything else. Down can be dry plucked before you put them in hot water and wax them. Yes, it will take longer, but you will be getting more out of each bird than you otherwise would have. If you don't want the down find someone who would appreciate it and give it away for Christmas or birthdays in small bags. If you want to be paid for your efforts, sell the down.
I know it's not for everyone, but we have a firm "waste not, want not" mentality when we butcher. If it can be used then we will use it. When you spend so long raising and processing an animal you want to get the most out of it. I've dried skulls and sold them to reptile owners as terrarium decorations, kept down and small feathers for pillows, peeled the feet for stock, the wings/wing feathers for crafts, etc. The goal is to make the compost pile as small as possible.