I think we have WoB on this, I'll see if I can find the quote.
The Shards could have created humans based on what they already knew while the original humans remained on their homeworld.
He can use illusion magics so his appearance isn't a fair indicator.
He has some method of forward time travel IIRC but he is also far older than he should be so still somewhat valid.
A
They wouldn't be born with the ability but they may be able to gain it, by eating Lerasium for example.
It's both that make the unique abilities I think.
On the tranquiline halls being another shardworld, that one has definitely been discussed, I can't remember much against it but I think Brandon has said that either Honor or Odium were not native to Roshar, can't remember which (May have been both)
On the second, we don't know that Odium isn't still on Roshar, besides his presence might only be necessary to give them the abilities, not to use them.
I'm going to go search for some supporting quotes.