3. I noticed that in order to mechanically back up the fictional positioning of your fallen paladin (which in my opinion implies much more than just being a traitor)... you had to house rule the character, now I find this a much more satisfying answer than "fictional positioning" (Since in essence you are just playing a paladin who can do whatever he wants and is not beholden to the archetype)... wouldn't this be just as viable in 3.x if you wanted to keep encounter balance equal? Just house rule a rebuild of the character or give them a number of feats equal to a fighter.
The reason why we went with the (obviously extended) retraining route into a rebuild was because the character did much more than just fall. He went from being a demon-exorcising/slaying crusader to a living avatar for a demon prince. It required pretty severe mechanical measures. Rebuilding characters mid-game has never been foreign to any of my games (regardless of edition). Not even 3.x (the most malleable system for character creation) allows changes that don't perturb the encounter budgeting (eg outright adding of a template changes the CR of a character until they've earned the XP to warrant the level-adjustment).
If you don't want to rebuild as a Blackguard (or Themes, PP, ED) or retrain with smaller components, I think you can pull a more mundane version of the "fall" off in several ways:
1) Divine sponsorship can just come from an opposing deity, more in line with the character's methods and behavior regime, with the fictional positioning representing that change. It could be subtle (such as refluffing the manifestation of the God's voice when invoking a Divine keyword power that buffs Diplomacy) or in your face (an actual agent of the deity arrives as ambassador). Given that Radiant isn't "Holy", it mutes the moral implications of its invocation such that evil deities, and their proxies, can, and do, invoke it the same as their good counterparts.
2) If you want a more dramatic thematic change, you can agree upon some keyword changes in various powers (such as Radiant to Necrotic, Cold, or Psychic).
3) You can leverage the Disease/Condition Track mechanics. Stage 1 can be something relatively mundane such as loss of a Healing Surge and that being narrated as feeling distance from your God. Stage 2 might be keyword changes to powers. Stage 3 might make things permanent. Instead of an Endurance check, you could go with a Religion check. When permanent, it may require some form of atonement quest as a Ritual component and some form of Ritual with a Skill Challenge (such as Remove Affliction or the Exorcism - can't think of the name right now - ritual).
4) You can leverage the open descriptor magic item rules. You can have Alternate Advancements (such as Divine Boons) that basically work as a template which would endow the thematic and mechanical components.
As far as the question of "can't you just temporarily turn a 3.x Paladin into a Fighter or rebuild as a Blackguard?" You certainly can ignore alignment with 3.x with respect to Paladin ethos. This is precisely the route that we went when a player in one of my old 3.x games willfully initiated a fall as a Paladin in order to become a Blackguard. However, in total, you've still got a considerable amount of work to do to fully detach embedded alignment from the supernatural mechanics of the system, primarily the impact of prolific Divinations on play (hello Paladin!), but also the Abjurations, Evocations, and the Conjuration rules.
With the Paladin class, in the end, we changed At-Will Detect Evil to be a (Su) scaling bonus to Sense Motive following the progression of the Ranger's 1st Favored Enemy (supernatural insight into the nature of people/beings). Then Smite Evil lost the "Evil" part, just becoming "Smite." Aura of Good got changed to something that I can't remember. Some other changes. Basically I all but rewrote the class using bits and bobs from various Prestige Classes.
