Hm... Here are some thoughts.
Well, and then you have denth as "varatreledees"...
But treledees seemed very hallendran, anyway. Not the sort to be secretly worshiping an evil shard, or trying to plant an evil cult off-world.
Lol this was funny. Still, it is an interesting dynamic. Sazed holds two shards, making him the most powerful entity in the cosmere. However, he is also quite new. All the other shardholders have been around for hundreds of years, and as far as we know they are all the original holders of them following when adolnasium shattered.
Huh. Quite possibly there is something down there... No idea what Sazed would put down there, though. The few people who survived the events at the end of hero of ages were accounted for.
This can actually be explained fairly well I think. The worlds didn't all develop completely apart from each other. They had the intervention of shards who came from the same place, plus a select few individuals worldhopping around planets. Some aspects of culture and such may permeate.
I believe Sazed specifically states in the words of founding that allomancy is of preservation, hemalurgy is of ruin, and feruchemy is of harmony (both ruin and preservation). This is because allomancy results in a net gain of energy (allomantic powers are fueled by preservation), hemalurgy results in a net loss of energy (some is transferred through the spike but some is lost), and feruchemy has neither a gain nor a loss.