Yes they should be returned as they are stolen and vandalized property.
As for the rest of your post-—just a theory. They were vandalized between 1801 to 1812 and in 1828 the Greeks became independent after a brutal war of independence wiht the Muslim Turk.
The Marbles would have been safe after that.
True. As we don't have a counter-factual; we can only speculate about the fate of the marbles, had they been left in place. We do know that the British took very good care of them — so, at least they continue to exist. Now, the only issue is where they should be.
No don’t think so. Lots of art all over the world. Little late to start returning it to point of origin. Be thankful the British Museum did such a wonderful job of preserving them.