Yata

We don't know Who was the First to trap a rival Shard.
I believe (but don't take my Word as fact, Just as a consideration) that Preservation trapped Ruin First.
Odium came to Roshar After:
- Searching Ambition without find him/her.
- Going to Sel and killing Devotion and Dominion
- finally find Ambition, Battle with him/her and kill the rival.
- go for Honor and Cultivation.
Only After all this events, Odium could be trapped on Braize.
If Preservation and Ruin didn't go around and doing stuffs before crafting Scadrial. The Ruin's prison is older.
By the way Ruin and Odium's prisons are really different. Honor didn't sacrifice himself to trap Odium and Odium isn't nearly as powerless as Ruin.
Ruin's prison was actually an auto-counter to every Ruin's action more than a prison. While Odium's One seems to be' a simple "you can't go away"
Edit: fixed some typos

I would add to Calderis's point that Alethela (the modern Alethkar) was the Silver Kingdom dedicated to the art of war and Taln is the Herald of War.
It's likely the Herald have almost fixed "spawning points" and from there they began to prepare the people in according with their roles.
 
About the Wet, I think (as other) that is caused by the same reason the Shardblades have it. Taln didn't simply arrived by Elsecalling. He will have no way to use the Elsecalling and much more I don't see him capable of walking for continental distance in the CR to arrive there.

This is an easy one, Kelsier there is a Cognitive Shadow not a fully physiscal being. this is the reason he may walk over the "cognitive sea" without fall below, they are made of the same substance.
You may notice how all the living beings we saw in Secret History don't stay on the "Cognitive sea" without trouble. Khriss and Nazh are camped on the "solid part" (a lake in the phisical) and they probably move follow the rivers/sea/lake and other water path.
Hoid on another side, need to ride a Cognitive Shadow to use it as a boat when He tried to go near the Well (no water path to go there) because he can't walk on the "cognitive sea" otherwise.
On the different looks of the CR, every Shardworld seems to have a peculiar look, based on a specific theme. But they are functionally the same thing

I have some news (or to be more precise I will have some news soon).
Someone (maybe from this forum) replied to my Tweet to Peter with a specific istance of MAG's Hemalurgy not equal from Canon Hemalurgy...And Peter said He would check this issues.

I will update this message as soon as I get a further answer

I actually manage to get a real answer with a real WoB here:
This mean that many Greatshell species have a setted Polestone as Gemheart but there are some species with a variable Gemheart

Apparently in those cases it's possible to ask help until the disadvantage remain.
Probably those "helper" can't start in the arena and of course in a 4VS1 probably a lesser Duelist would fall before the help arrive...Much more, also if Adolin had help if He was defeated the fight will be over (Renarin and kaladin's status aren't relevant to the match's outcome).
Maybe there is also the need of a formal request of help to the judge to made the helper join the fight.

Also moderate invested object would stop a Shardblade.
I asked to Mister Sanderson if a Metalmind would stop a Shardblade and He said yes. (then my trip to a Tin Ferring fighting a Shardbearer with his SmellMind-Sword XD)