True line of sight, so since the Slann is up on his floating palanquin, he can see over the heads of most things. However, magic missiles specifically can't be cast if the Slann's unit is in combat. But he can cast direct damage spells, which can also be cast using the skink priest's Arcane Vassal rule.
The new arcane vassal rules do allow for a magic missile to be cast through a vassal if the slaan's unit is in combat providing that the vassal's unit is not in combat.
That is correct. The slaan can join any unit in the army or move out of TG if he wants to during the game.
damn. I always used an empty 50 mm base to prox the slann thanks to the rule which specified the large target line of sight. Now i have to buy the model or my opponents won't let me cast spells... (no money )