But still time always went forward never back and forth between the worlds.

I'm not sure we can eliminate the possibility, though. Lewis probably didn't think about this, didn't think it through. Without his explanation, we can't really eliminate any possibilities. It is a fantasy story, after all...

There is a muddle of time between our world and Narnia, so we don't really know. I think it's possible that the pirates lived on the island for a while. C.S. Lewis doesn't really specify what happened on the island. I'd imagine that they would need to populate a bit and learn how to depend on themselves before being thrown into another world, where they would again need to depend on themselves.

I'd also thought the first Telmarines were the descendants of the original pirates, but on checking it out it says quite clearly that the men who fled to the cave were some of the original pirates.

This is problematic, because although there may have been some local pirates operating in the Dutch East Indies (Indonesia, not India) there certainly weren't many nor were they doing that kind of noticeable action.

In those days the area was probably as close to being pirate-free as it's ever been- certainly more so than now- because the waters were patrolled by the navies of the Great Powers, particularly Britain, and they took short shrift with pirates- they weren't overly worried about offending the local population or worrying about their rights!

So, yea, he probably hadn't worked out the timeline when he wrote PC.

And it sure sounds like he had the "Bounty" mutineers in mind:

In 1790, nine of the mutineers from the Bounty and Tahitian companions (six men, 11 women and a baby), some of whom may have been kidnapped from Tahiti, settled on Pitcairn Island and set fire to the Bounty.[9] The wreck is still visible underwater in Bounty Bay. The ship itself was discovered in 1957 by National Geographic explorer Luis Marden. Although the settlers were able to survive by farming and fishing, the initial period of settlement was marked by serious tensions among the settlers. Alcoholism, murder, disease and other ills took the lives of most mutineers and Tahitian men. John Adams and Ned Young turned to the scriptures using the ship's Bible as their guide for a new and peaceful society

Ned Young then died of asthma, leaving John Adams as the sole male, with nine women and a large brood of children.

It's the usually muddle about times, Pole...