I'm new, too. Haven't even bought my first duck. I'll answer the questions I can:
1. I'd love to get an answer to that!
2. I think they should be paired and will stay that way for several years. I would guess that if you buy them individually, you'd need a 50/50 split between males and females; maybe a couple of extra females. I think you were thinking chickens where you need more hens than roosters.
3. Even if they're monogamous, they often breed across lines. If you want to maintain purity, separate them from the other breeds. However, most of the extended Mallard family are very compatible except during mating season. Even when you deal with a licensed hatchery you can get Hybrids and you're completely unaware, so the only way you can be sure is breed your own.
Sorry it's not much info, but I know how you feel attempting this when you don't know what you're doing and have no one to ask. I quit worrying that my questions were stupid weeks ago!