Of all the theories I have read here, (I skipped some pages, say sorry, say thankya) all of them seem to overlook two key points; the will and the watch. Ka is a wheel, and how many times did someone say "I set my watch and warrant on it" in the series? This is the biggest swerve of them all.
I think (Feel free to pick me apart).....
Roland was wrong. Time is only one-way in Keystone Earth, not Mid-World. Every time a new threat to the Tower emerges, Roland re-emerges, forms a new ka-tet, and is utilised by the Tower to right the wrongs perpetrated upon it.
Calvin Tower's ancestor's will is the key to my theory. How else would someone from over a century ago know Roland's name, unless they had dealings with him? The family lines may stay intact, and even bridge the two (or more) worlds, but the individual players change with the exception of Roland. Once Roland fought off the plague of the Crimson King, Eddie and Jake were absorbed into Calvin Tower's family line (Their new surname, Toren, was Tower's ancestor's original name), and presumably, Eddie and Susannah bore another child into the bloodline, who would either serve, or bear progeny of its own, to serve in Roland's Ka-tet. Roland and Randall are immortal servants of the Red and the White, and every incarnation of them results in the same outcome. When Roland finally reaches the Tower with a) both guns b) all the artefacts of Eld, and c) his ka-tet intact, then the Tower will be truly saved. The watch started losing time once Susannah started having her dreams; the proximity to the Tower was a coincidence. The reason Roland was so afraid of her leaving, though he did not know it, was that the final break in the ka-tet would doom him to repeat his trial. When he reached the Tower and sent Patrick on his way, the watch started going backwards, in effect resetting time. The watch's batteries were guaranteed for 50 years, a big deal was made of this point. The watch wasn't malfunctioning, it was literally resetting time in Mid-World. Had the narrative swept to, say, Lud, I am sure we would have eventually seen the poison cloud shrink, and return to the canister from which it was released.