I know I can sound a bit straightforward when I argue about something, so please do not see hostility in my words. I simply think your call here is the wrong one, so no offense.
Resistance to Pain doesn't 'sound' like Toughness, it sounds like Resistance to Pain. A modifier that would simply help resist from the effects of pain directly, of pain already there, not the ability to resist receiving such pain in the first place.
So... according to you, the examples on that page (your only reference for this, byt the way) are all cheating 'death', are all immune to Seduction, fear, molecular manipulations etc? They don't need to breathe, eat, sleep or drink? They all can project thoughts?
Grommash (World of Warcraft)
Gregory Kennedy (The Young Guardians book series)
Sauron (The Lord of the Rings)
Raleigh Becket (Pacific Rim)
Stacker Pentecost (Pacific Rim)
Will of D Members (One Piece)
Haoshoku Haki users (One Piece)
Lucifer Morningstar (Vertigo Comics)
Green Lanterns (DC Comics)
I'll be honest, I don't know much about the others, but I know of the Will of D guys, the Haoshoku Haki users, and the Green Lanterns. Donquixote Dolflamingo is stated as having an inherent fear of the Will of D. members. He almost died of hunger in his life when he ceased being a World Noble along with his family. Monkey D. Luffy (and most likely some of the others Will of D. Members and Haki users) has been shown eating a lot, sleeping a lot, and not only because he can. He has stated himself being tired and hungry multiple times. The Green Lanterns were eating, sleeping and breathing (unless they were an alien species that didn't need such things in the first place) before they became Lanterns, and Hal Jordan sure as heck has been seduced before and manipulated as well, not always to his advantage. Seems like your so-called Immunities are better known as "Defense", as in "Mental", and that Self-Contained Lifeform is false. Oh, and Hal Jordan, the guy with the Most powerful Willpower in DC Universe... has been specifically noted to do experience fear, sometimes a lot of it. He simply 'overcomes' it when that happens.
Big difference.
Speaking of Luffy or the Lanterns, never projected their thoughts either, not before the latter got their Power Rings anyway. While Luffy has been showed displaying formidable versatility with his powers, they were all Devil Fruit related. Nothing indicates it has anything to do with his willpower, it's simply creative use of his powers. As for the Lanterns, again unlike they come from an alien race already having that ability, there is 'no' metamorphosis going on, be it actual Metamorphosis or simply boosting abilities. Not until they gain their Rings.
Finally, cheating Death? Let's see...
Gol D. Roger, Portgas D. Ace, Whitebeard, God knows how many Green Lanterns... oh, and Sauron, multiple times... whose only reason for not being more permanently destroyed was the One Ring holding most of his power.
...all dead.
They 'cheated death' for long because they were badass and powerful. Their will must have helped, but it didn't give them a magical way to mysteriously avoid it.
One, I think you're seeing "Immunity" in what has been repeatedly proven to be simply high resistance, and 'common facts' in what is barely rare occurrences that can be explained with 'because the plot demanded it', or perhaps disagreeing writers..
Two, the very examples of your own source go against your very source.
Three, you only have... one source, which is a wiki that was written by someone. It is possible that person misread it, or decided strong defense meant immunity. Now, if there was more than a single source supporting your claim, I might reconsider, but until then, nope, I don't see any of this. And with all due respect, I think you see what you want it to be rather than what is actually there. If you want something like what you describe, look up Power Cosmic/Limited Phoenix Force. But Indomitable Will does not grant those things as far as I'm concerned. It's merely a part of its sum.
I've said enough on the matter. If you don't agree with me a this point, you never will, so further arguing would be useless. ^_^
See you around.