Trying to take over the cage for the "greater good" would not necissarily be any more good than wanting it for personall gain. It "might" be more lawful/orderly, but it would not be any more right or good, at least in
AD&D terms. Most lawful evil beings are after their there version of the "greater good".
Selling someone into slavery is not necissarily more evil than killing them. It could reasonably be argued that it is more "good". Atleast if they are alive they have a chance, however small.
No one fits their alignment perfectly, except superfical and unbeliveable characters. Alignment is just the closest value that fits the tabulated sum of a persons' overall actions and values.
I my games NPCs and PCs play by the same rules. There is nothing a PC could concivably do that would be totaly impossible for an
NPC and vice versa. It's self balancing. I don't run or play overpowered games for the most part. Anything is possible, but everything significant must be earned. Balance simply for the sake of balance is pointless, in my opinion.
Hiemdall is a sworn enemy of Loki. Loki often uses giants in his schemes. Rowan Darkwood's species enemy is giant. Darkwood could have defeated, or foiled the plans of some powerful giant in service to Loki, earning Hemidall's gratitude. It is not inconcevable that Heimdall could have granted Darkwood priestly powers, without all of the typical ethos restrictions, as a reward, or as the agreed payment for his services. Powers/Dieties/Gods are certianly powerfull enough to allow their power to be used by any damn person they choose.
I would not bar a PC from reciving a similar boon in a similar situation, just becasue the rules, wich are specifically stated as guidlines, say this is not typical. They would still have to learn to use the powers granted to them, (
ie gaining levels) and if they did something to anger the source of their powers, or violate the agreement in wich they gained the powers, then the powers could be removed.
As for the ranger abilities, I personally see no reason to remove them from someone for failing to follow it's guidliness, as virtually all of the abilities are mundane (a paladin's abilities on the other hand are nearly all granted by a god and would lose every single one upon failling). I would certainly remove any priest spells and possibly reduce the other abilities of a fallen ranger, but would not remove them. I see no balance problem with this as the character is simply a fighter who needs more experience to rise in level, has a better chance of hiding/moving with stealth, and who cannot specialize or gain mastery in any wepaons. This is effectivly what Darkwood is. He would be MORE powerful as a fighter.
Planescape is not significantly diferent form most other setting in the way powers/gods work. Indeed, it's all a part of the multiverse. In away every setting (at least every offical one) is part of the planescape setting. Rowan Darkwood himself was a great hero in a rather obscure area of the realms. He participated in the Bloodstone wars and helped save the area (and possibly the world) from the Witch-king Zhengyi and the Tannar'ri Lord/God Orcus.
Priest of nonspecific higher powers are rare pritty much everywhere and are not barred from the realms (though they would probably not be natives). The standard cleric (as per the 2E players handbook) is stated as possibly serving simply "good" or "evil", not necissarily a specific god.
I used 2E references as the vast majority of offical information on Planescape and Rowan Darkwood are in 2E sources. If there are specific considreations of 3/3.5E then they might override a few things. If you are using this in a 3E game that is.