The Lich King is his scars, though.
Assuming every energy can affect all but their polar opposites, there shouldn't be a problem Lightforging the Lich King, as his power is Death, not Void. Might lose connection and command over the Scourge though, we've never really seen any Light affiliated necromancers.
Also, this topic make me sad thinking about that one paladin in Icecrown, we tried so hard to save him from undeath. Now years later, we finally found the one thing that seemed it could have saved him.
I think it could heal the human Arthas and make him normal again, i.e. remove the Frostmourne curse. I still think you would need to remove the Helm of Domination though. I think that if a Prime Naaru tried to cleanse the Lich King in all his armour and stuff then the Lich King would just be killed. The way I see it is that the human Arthas is the cursed vessel, infected with death(?) magic. Likewise Illidan had fel. The Naaru could purge that. I don't think the Naaru could make the Plate of the Damned holy.
On another note, was Arthas undead or just magically cursed? From memory he didn't die until the ICC raid.
As for Bolvar, the same would apply. I think he could be healed and brought back to normal or at least some lightforged state. Again this would require removal of the Helm of Domination.
On another other note: wouldn't it be cool to have some Caverns of Time dungeons where instead of the Infinite Dragonflight trying to affect events in the past we instead prevent events from happening in the present/future such as this? Blizz has shown that they like experimenting with alternative themes to heroes, for example Knights of the Frozen Throne in Hearthstone or all the skins in HotS. This could be the same in WoW. They would not fun, artistically different dungeons with no impact on the story but still lore-relevant.
TO FIX WOW:1. smaller server sizes & server-only LFG awarding satchels, so elite players help others. 2. "helper builds" with loom powers - talent trees so elite players cast buffs on low level players XP gain, HP/mana, regen, damage, etc. 3. "helper ilvl" scoring how much you help others. 4. observer games like in SC to watch/chat (like twitch but with MORE DETAILS & inside the wow UI) 5. guild leagues to compete with rival guilds for progression (with observer mode).6. jackpot world mobs.
if you could get Bolvar from the helm, but I doubt the Lich King would allow it
I am not really sure if a prime naaru has enough power to pull that off, not to mention X'era was the last one.