Power keys and spell keys, total mess

Spell keys can permit arcane spells to ignore dimensional pathway restrictions.  A wish, it's safe to say, ignores just about all restrictions.  ;^)

The planetar is converted in the SRD, casting divine spells with a caster level of 17.  Raise dead is one of the planetar's spell-like abilities.  It is a 5th level cleric spell, castable by a 14th level caster, and a planetar is at -3 caster levels on any Inner Plane, so the planetar would be able to cast that spell.  On the other hand, it would not be able to use its earthquake ability (an 8th level spell), on an Inner Plane or on a plane more than three planes around the Great Ring from its home plane. 

To determine whether a monster can use a particular ability on a given plane, check out its 3e conversion.  Use an official or PSCS conversion if you have one available, or if you need to, convert it to 3e personally.  Spells and spell-like abilities will be affected by diminished caster level; supernatural and extraordinary abilities will not be.  The 3e version of any monster will have all its abilities given one of these labels.

The Prime Material is an exception to the normal rules. Divine casters don't lose any spell levels there.

Union is in a demiplane (called Haven), which in the Planescape cosmology can reliably be found in the Ethereal Plane (though there are a few exceptions, known as half-worlds, connected to  Yggdrasil and other planar paths). It loses one point in the Ethereal, two points on the Prime, three points in the Astral, and four points in the Outer Planes. Bad place to build magic weapons. Though I don't use this rule, personally. I don't think it adds to the game's fun. 

The Astral doesn't have demiplanes - those are a feature of the Etheral specifically.

 Now a clever Mercane in a setting that still uses these rules would make sure his shop has as large a series of portals as he can afford to all over the place. And simply build a forge on the opposide side of each portal. So you can buy a sword from the Astral wing, or the Prime, or the Inner Planes, without having to have his blacksmiths do more than walk into the next room down the hallway.

Where it's enchanted.  If it has been re-enchanted, use the latest plane.

As for your comment on caster level not affecting spell-like abilities, it seems you have a clear interpretation which can be applied consistently, which is all that's really needed.

Huh... The Planewalker's Handbook doesn't say, and I don't know where else one might seek that information.  I suspect the possibility was overlooked.  As a house judgment, I would rule that those spellcasters never lose levels!  Lucky them.

Now the bad news: they don't have powers to bend the laws of reality for them, so I would say their divine spells suffer from dimensional pathway restrictions, at the very least, and possibly suffer from all the same restrictions as arcane spells, depending on how you want to conceptualize philosophy-driven magic.  This might open up the possibility of a rare sort of "divine spell key" that could mitigate those restrictions for those casters.  I would think it would be very likely that such "keys" would be behaviors in line with the philosophy, like a believer in death having to, say, eat some meat for a low-level spell, and sacrifice a live creature for a high-level spell.  On the other hand, you might want to ignore all of that and simply say that someone dedicated to a philosophy is in a better position when it comes to such spellcasting.  After all, the Athar priests of the Great Unknown don't seem to be mentioned as suffering from dimensional pathway restrictions, only from the lack of power keys.

Another option I just thought of would be to assign the caster's home plane as his "base" for counting level loss. 

Reduction in Power for divine casters and reduction in magic weapon "pluses" were actually my least favorite element of 2e Planescape. The idea behind it is a good one but in practice it's just a huge hassle. I usually make the loss of power less severe or just do away with it entirely in my games. Spell keys aren't actually too bad and modified spells can actually be fun. Gives a change of pace for the casters in the party and encourages them to use the new spell variations creatively.

Belief is a powerful thing and may itself give the ability to ignore dimensional pathway restrictions, as a standard feature of divine magic.  Now, the fluff text says that it's powers who bend the rules for their worshipers, but as has been pointed out Athar clerics don't seem to suffer dimensional pathway restrictions from not having anyone to bend the rules for them, so perhaps the narrator of the fluff text isn't fully accurate.

In general, I think philosophical clerics should either be assigned no level loss anywhere (which puts them in a favorable position with respect to other divine casters) but suffer dimensional pathway restrictions (balancing out the favorable position somewhat), or should count their levels losses from a specific plane like anyone else but not suffer dimensional pathway restrictions.  They don't get power keys handed to them, so on the whole they're slightly penalized.  As a slight compensation, the plane from which they draw their power should probably be fairly centrally located.

The Athar count their level losses starting from the Astral, the Believers of the Source from the Ethereal.  It would certainly stand to reason that elemental priests such as Athasian clerics of Rain (Water) would count their level losses from the associated Inner Plane.  Druids might collect power from the Outlands regardless of their alignment, or the neutral major plane of their alignment (possibly their choice), or, being connected to a more vital component of the cosmos, might draw power from the Prime Material.

One could also assign a philosopher-priest his own plane of birth as a base plane, or a plane that features his alignment.  However, I rather like the previous idea.