Some of this tweaking will occur during the beta. This is still very much a work in progress. Yes, the Night Elves and the Undead have advantages at night. In single-player games, the heroes may carry over from missions to mission. But in multiplayer games, the heroes will be chosen and built by each team member or adversary, and they will not carry over.
Items pop into the inventory in the order found.
Top left, top right, bottom left, then bottom right. If you decide to share an item with an ally, you click on it to drop it, and the other items shift up or over to fill the empty space that is the result of the item's removal. Yes, but almost certainly not the same spell. Early in testing Psionic Storm during the StarCraft beta, we were able to layer Psi Storm over and over again. They fixed it.