Extesian

I think its simply that Shards have access to enormous knowledge through Connection but the Vessels do not have minds capable of understanding it all, they don't just suddenly know everything they can know. I picture it as taking a huge amount of time for the Vessel to explore those Connections to understand what it all means. 
 

I may not be Calderis, but
1. Size does, I think Sazed talked about a ring being full and Brandon has talked about metalminds being 'full'. I can't see that shape would?
2. Should be no difference by this
3. Same as two i think

My understanding is that it can be fought, or at least that it is less likely to happen unless the Listener (whatever form they are in) is open to it. This quote is quite informative. 
Rlain was pretty impressive so I suspect he just fought it. 

Good question! Luckily we have a recent statement from Brandon!
So it was the nature of the Shard and the fact that Ambition is, understandably, a strong intent to rival Odium.