Just because we've never seen the Sun Rune effectively wielded by one person doesn't mean it cannot be. I think it is no coincidence that the Sun Rune takes a head slot, the Dawn Rune takes a RH slot, and the Twilight Rune takes a LH slot. It seems plausible that one person could (potentially) bear all three at once. Since the two lesser Runes exist to keep the Sun Rune in check, that could be enough to keep the bearer sane. It's all conjecture, I admit (there's no evidence that one person can bear all three Runes at once), but it's plausible, at least.
And FYI, the "True Godspeed" and "True Swallow" Runes are not True Runes. Neither are the Dawn and Twilight Runes.