They would not stack
It's covered by this sentence:
In most cases, modifiers to a given
check or roll stack (combine for a
cumulative effect) if they come from
different sources and have different
types (or no type at all), but do not
stack if they have the same type or
come from the same source (such as the
same spell cast twice in succession).
If the modifiers to a particular roll
do not stack, only the best bonus and
worst penalty applies. Dodge bonuses
and circumstance bonuses however, do
stack with one another unless
otherwise specified.
Emphasis mine.
Note, that this is referring to Bonuses, but I believe the same applies to Damage as well, unless the spell says it stacks, it wouldn't stack.
Similar Reference in the Magic Section:
In cases when two or more identical
spells are operating in the same area
or on the same target, but at
different strengths, only the best one
applies.
Which I think covers the damage stacking aspect.
For Vision, Daylight and Bright Are The Same
For vision and hiding purposes, there are only three levels of light, as you mentioned. Bright, Shadowy, and Darkness.
"Bright" in this case means it's bright enough that there is no hindrance to vision at all. "Shadowy" grants concealment, and Darkness creates effective blindness if you can't mitigate it.
So yes, for these rules, a torch you're holding and the sun are the same thing, within the "bright light" range of the torch (20'). Outside of that range, the torch stops providing bright light. Get some distance on someone with a torch and you move into shadowy illumination against them instead. (If you have different vision modes than they do, you could actually have different conditions to see them then they have to see you, depending on the light.)
example:
If you're holding the torch, you're in bright light and can't Hide without Cover.
If someone else is holding the torch and you're 5' away from them, you're in bright light and can't Hide without Cover.
If someone else is holding the torch and you're 25' away from them, you're in shadowy illumination. That grants you Concealment, and you can Hide.
The same rules apply for any light source, you can use the Vision and Light table for the effective ranges of different light sources.
So what's Sunlight do?
The sun creates both bright light and bright sunlight. The difference is only in the case of monsters or effects that mention something related to that, like an Orc:
Orcs are dazzled in bright sunlight or within the radius of a daylight
spell.
A torch doesn't generate bright sunlight, so in this case it's different.
This Creates Lots of DM Interpretation
As you noticed, this gets confusing pretty fast when Shadowdancers get involved. Just what is "some sort of shadow", and where is it? Ask your DM. It's relative to the light, the position of the light source, the size of the thing casting the shadow, and the rules don't have anything to say on the matter whatsoever.
How big a shadow do you need to use it? Doesn't say. How much shadow is enough shadow to shadow jump? Doesn't say. How long is the shadow being cast by the enemy in front of me? Doesn't say. (This is probably why Pathfinder changed the wording on some of these abilities to be near "dim light" instead, which is more of a known rules thing than "some kind of shadow.")
As a DM dealing with a player who uses these abilities, it can get pretty confusing to try and sort out. It's actually easier in a dungeon with no light of its own, because if some player is carrying a light source, we can map it out on the board pretty easily relative to them and see where the shadowed areas would be (and which direction the player shadows are going, if you want to hide in the shadow of the Wild Shaped Druid).
But in an outside area at 5pm? Where are the shadows in that? It's a lot of work to sort out exactly how it all works, and it's often easier to come up with a simple rule of thumb and apply that rule of thumb consistently.
Hide
The one exception you mentioned is "how dim is dim enough?" In order to use Hide, you need concealment. Shadowy Illumination provides that. So you need to be in Shadowy Illumination, which is going to depend on what the area's light sources are (but most of those have a radius of providing light, and the table in the first link I provided has those distances).
You also need to be not being observed. If they're watching you, even with Shadowy Illumination, you can't use Hide. "Watching you" basically means they can see you at all, because in D&D vision is omnidirectional: characters are looking in every direction on every turn.
That's part of what makes Hide in Plain Sight so good (with HiPS, you can use Hide while being observed).
Best Answer
I'm sure there will be good answers for how to houserule this spell and bring it under control, so I'm going to go on an extreme tangent and suggest something else entirely:
The cleric clearly wants to be a rockin', evil-burninating machine. Awesome! Roll with that. Bring out more, nastier evil outsiders and help make that cleric player feel awesome. As the DM, sometimes countering the spell's effectiveness is going to be what delivers the most fun. Sometimes throwing hapless demons at that Celestial Brilliance just to watch them burn like moths in a candle is going to be most fun.