John, I think you are well on track, but I'm dubious about basing detailed conclusions on oral tradition, especially when we're so poorly able to evaluate that tradition.

We know that shantying as so many writers have described it over the past nearly 150 years was the usual practice in British and American merchant sail by the time of the Civil War. Whall and Robinson both went to sea in 1859-60 and reported years later that shantying was then common. The 1868 shantying article confirms that shantying was widespread in the 1860s.

The oldest sailor singers recorded by Carpenter went to sea in the 1850s. AFAIK, none of them suggested that shantying was a new development that took off later.

So we can be certain about the '60s and virtually certain about the '50s.

Things get murkier earlier than that because of the dearth of sources. The few sources that we have give a handful of titles, and a surprising number of these (in Dana) are no longer identifiable.

This suggests to me (and I assume to writers like Colcord, Doerflinger, and Hugill) that most of the best known folk-revival shanties either had not appeared by, say, 1849, or else had not yet circulated widely. Since they have such good tunes and so many shantymen were at work, it would surprise me if they hadn't spread relatively quickly once they came into existence.

It also suggests to me (though the evidence is extremely thin) that there were far fewer "established" shanties in Dana's day. In other words, singers were creating shanties, but individual repertoires were not widely shared and the tunes had not often become so melodic as to spread rapidly or be remarked on by writers. If that were true, it would point to the period around 1830 as the dawn of the shanty. The advances in shipbuilding in the 1820s that many writers have cited as a major influence on the rise of shantying fits well with these educated guesses. (Though they're still just educated guesses.)

It's mainly wishful thinking, IMO, to assume that there was a very long tradition of shanty making in English before writers began to comment on it in the 1830s. Admittedly there were fewer writers then than now, but my guess is that if early shanties had truly memorable tunes (like "Highland Laddie") they would have attracted more attention. Remember that Dana mentions them because, unlike most writers, he had actually sung them at work. They were impossible for him to ignore!

When it comes down to the time when specific shanties became common, I'm afraid the limited number of contemporary mentions leaves us mostly in the dark. (We know even less about which now "indispensible" lyrics were sung.) The limited evidence shows that most of the best shanties were well known by the time Harlow went to sea (on his single voyage) in 1876-77 and a good many of them at least ten years before that.

I concur with Gibb and with Bullen that except for the very few strongly narrative shanties, after the first stanza or two anything could be sung, though I'd imagine that individual shantymen often fell into the patterns they were used to. The overwhelming evidence that solo lines were more commonly repeated than paired up (which is not ncessary to hold a work gang's attention) suggests just how common unrhymed improvisation could be. Shanty singing was not a musical performance. It was just a way of getting work done.