How will the PCs train?
We'll be playing by the new Player's Handbook rules which states that characters don't need to formally train until they hit level six. Since the first island is designed to take a party of six PCs to level four, that shouldn't be an issue. By that time, the company in charge of the expedition should have made a few arrangements for formal training. In the case of some classes there may be other opportunities right away, if they know where to look.
Can the PCs leave the island?
Yes. No one is forcing them to stay. But booking passage back to Reanaaria Bay won't be easy. That's also not discounting the potential honor hit for choosing to take a boat ride when they are supposed to be exploring. Better have a good reason to go back to the mainland if you're going to do so.
Will fighters have the opportunity to become knights and/or paladins?
It's an idea I am knocking around at the moment. The question is why would a knightly order go to the archipelago?
The current party is taking part in the Reanaarian expedition. What about the Kalamaran and Svimozhian ones? Will PCs be allowed to be from these groups?
As of right now, the Reanaarian expedition is the only one that has managed to set foot on the island. By the time the PCs head out, there is no word of the other two nations landing on Malakoah. That may change in the future. It will be difficult to justify explorers from rival groups teaming up. Though in the event of a TPK (total party kill) I may allow the players to create an new party based on one of the other expeditions.
Why has this song been stuck in your head all weekend?
Will latecomers be allowed to join the game?
If I have room for a few more due to attrition. No "waiting list" as of this time.