They don't wait until they are full. If they can load anything, then they will leave.
Correct
Correct
No. You cannot harvest a planet owned by another player. The distinction is minor, but while trading and harvesting are performed the same way, you cannot use the Harvest command against another player's planet, you have trade with that other player.
1. The base time is 60 seconds to load or unload, regardless of ship size.
2. Yes.
3. Entirely skill dependent. Basically one class of skills gives you a bonus in the amount of resources you receive from the other player. A different class of skills allows that other player to cheat you out of what you get, either by reducing what you actually get, or by giving him a larger trade.