I think all the theories you mentioned are plausible. Who knows if we'll never know the answer for sure, but what I took from the varieties of Hepatitis is that the man has just been subjected to that many dangerous things during in his life where modern medical care isn't available, yet he's so tough, his body has endured it.
As far as the radiation, I first thought of the portal travel, but you've got me wondering more on the stuff left behind by The Old Ones.
Thanks for bringing this question up. I'm gonna have to think on it some more.
As far as the hepatitis B...I'd be surprised if Roland
wasn't still getting some.
With the exception of Susan, we saw him get with both Allie and the *woman near the end of the series out of pure physical necessity.
*(God help me I can't recall her name. Imagine, you're the last person on Earth to bang a Gunslinger, yet no one remembers your name anyway. Shame. )