The idea of a 90 degree cone must be something from an earlier edition. There's nothing like that in 3E.
As for the matter of line of sight, it really is not very clear, but it does say that you have to present your holy symbol. So, you might infer that the reason they are protected by cover is that the holy symbol is not within THEIR line of sight. In other words, the undead have to be able to see the cleric. Even if they are "looking the other direction", they would still be treated as being able to see the holy symbol, since there is no mechanic, such as "facing" to determine that sort of thing. (I guess maybe they are somehow a holy symbol has a way of drawing their attention . . .). All of this, of course, is mere inference. I have no proof that this is the way you have to do it.