I can offer much info on this, since I spend most of my gaming time on steam and love to collect badges and achievements.
Here is my profile page, feel free to add me! http://steamcommunity.com/profiles/76561197993146044
Devs cannot specify the method of trading card drops (although this may have been changed by now..?); they drop regularly during time spent in-game (usually an hour or two each) and each account may only receive half of the total number of cards for that game. If you can now though Nifflas, I recommend keeping it this way, rather than rewarding them for game completion events, since the achievements are for that anyway. However, they can obviously decide the number to have. You may think the more cards a game has, the more profit the developer will make, but unfortunately increasing the number decreases the value of each one (since more drop for the players). There are usually 5-10 cards per game at prices of 15-50c each, depending on the game's popularity.
Badges work as LP Chip says, but the set of cards stays the same (so Nifflas only has to design one set rather than five). Foil versions drop at a rate of 1% and can form a badge of one level only. When you craft a badge, you receive a random coupon for another promoted game, a background and an emoticon that pertains to the game. This means Nifflas will have to design (or get people to design) backgrounds and emoticons for KU! Having said that, I highly recommend using game locations for backgrounds (Ljus etc.) and characters and/or items for emoticons, and I suggest having 5 of each (3 common, 1 uncommon and 1 rare is the norm for indie games). I also like the idea of having artifacts as badges (and maybe the enigma or katie's foot sandwich
for the foil badge?) Trading cards can, however, be random and include examples of different types of things in the game.
I hope i've helped somehow...
Really looking forward to this!