First, one of Brandon's earlier posts implies that anyone can burn atium, much like anyone can burn Lerasium. It was something along the lines of "all Obligators who weren't Mistings or Mistborn were given atium and stressed until their bodies burned it." It may, of course, have been a typo, but that would certainly be a twist.
Second, as for the other four Shards: Why has no one mentioned the Iridescent Tones (or whatever their name was), the things that send the Returned back? Lightsong's eventual remembrance of his time on the other side implies that there is an actual sentient power there. Let me get the quote.
The God King in prison. Lightsong had seen that too. But above it all, he remembered standing on the other side of a brilliant, colorful wave of light, looking down at the world from the other side. And seeing everything he loved dissolve into the destruction of war. A war greater than any the world had known, a war more deadly--even--than the Manywar.
He remembered the other side. And he remembered a voice, calm and comforting, offering him an opportunity.
To Return.
I would be surprised if it turns out it isn't a Shard, actually.
Archanon, that same idea of atium crossed my mind, but my understanding of Atium is that The Lord Ruler replaced silver from the allomantic table with atium when he had the power of Preservation. In other words, atium was changed from a everyone metal to a only-Seer's/mistborn metal.
Otherwise all of the aristocrats would've burned atium when the Steel Inquisitors "spiked" (tee hee) their drinks.
Since Sazed now holds Preservation's power, I would expect that he could change it back, thus making silver a useful metal, and atium a metal all could burn. (especially since he probably won't give his body away)
Also, about the day length, Sazed mentioned a people called the Nelazan...
"I think I have the perfect religion for you," Sazed said, his normally stoic face revealing a glimmer of eagerness. "It is called 'Trelagism,' after the god Trell. Trell was worshiped by a group known as the Nelazan, a people who lived far to the north. In their land, the day and night cycle was very odd. During some months of the year, it was dark for most of the day. During the summer however, it only grew dark for a few hours at a time."
Final Empire, hardback, pg 156
This shows that days in Luthadal are somewhat normal, because Saxed finds it strange to have long days and short nights. This means that Luthadal is pretty far away from the north pole.
Zas