For spells which affect multiple specific targets, such as 'Slow' or 'Mass Inflict Light Wounds', does the caster have to be able to see all the targets? Since it's specific targets, my assumption is that the caster must be able to see all targets.
e.g., if there is one target visible, and two more targets are round the corner (maybe 5-10ft away) and out of sight, then only the first target can be affected.
If the above is correct, then what if a spell such as clairvoyance is used to allow sight of the 'hidden' targets? The second spell is not being cast 'through' the clairvoyance, the divination is just being used to be able to see the targets round the corner which are otherwise in range of the spell.
Would this be a valid use of clairvoyance (or a similar spell)?
Generally, as long as you have line of effect from the origin of the spell to the target, you can effect a creature you can not see. But since Target spells originate from you, you must have line of effect to each target you wish to affect.
Clairvoyance doesn't help at all.
Line of Effect: A line of effect is a straight, unblocked path that indicates what a spell can affect. A line of effect is canceled by a solid barrier. It's like line of sight for ranged weapons, except that it's not blocked by fog, darkness, and other factors that limit normal sight.
You must have a clear line of effect to any target that you cast a spell on or to any space in which you wish to create an effect. You must have a clear line of effect to the point of origin of any spell you cast.
A burst, cone, cylinder, or emanation spell affects only an area, creature, or object to which it has line of effect from its origin (a spherical burst's center point, a cone-shaped burst's starting point, a cylinder's circle, or an emanation's point of origin).
An otherwise solid barrier with a hole of at least 1 square foot through it does not block a spell's line of effect. Such an opening means that the 5-foot length of wall containing the hole is no longer considered a barrier for purposes of a spell's line of effect.
Okay, it's not obvious to me that "Line of Effect" applies to all spells. It reads more like some spells are 'Target', some 'Effect', some 'Area' and some have a 'Line of Effect'. Under 'Target', it only states that you need to be able to see or touch the target.
However, I can't find any spells which state 'Line of Effect', so I'm willing to believe that it is a generic requirement. It does make sense, and would also rule out using a mirror in the same situation.
It also implies that for a spell like 'Blade Barrier', it is necessary to see all parts of the barrier when the spell is cast (I've been allowing players to target one end point, then just run the rest of the barrier out of sight until it hits a solid wall). Since the cleric has no 'line of effect' to the other end, they shouldn't be able to do this.
That's going to change some tactics.