the [confusion of] pronunciation of, respectively, 'often' and 'orphan', which suggests that in the late 19th century both words sounded similar...

Part of that (I think?) is that the pirates speak in a lower-class accent, so that the "of" of "often" would sound like the "or" of orphan.

But -- one of the corners of my junk drawer memory has an item about often:

The t was added during the inkhorn days to reflect some etymological concern of the inkhornists and was not to be pronounced. Pronunciation of the t was a hypercorrection that has insinuated itself into a state of inertial correctness.