In terms of playing as one. And by "Falmer" I mean snow elves, but saying as much in the title may be a spoiler.

Currently, I see two mods on Nexus that make the SnowElfRace from Dawnguard playable:

http://skyrim.nexusmods.com/mods/22037 by iM1TCH
http://skyrim.nexusmods.com/mods/21936 by TacobananaMan and SONNYSPAK

They seem to have vampirism working, but not female snow elves. And both mods edit a whole bunch of armor addons; I wonder if there's a better way to make apparel usable by snow elves.

Each of them also comes with a "RaceCompatibility.esm" file like many other race-related mods, but the files don't match so I don't really understand how that would make things compatible.

How do you create a mod like this and make it exhaustive, compatible and bug-free? I'd like to see a more organized approach for this.