Rather unexpectedly, certain ties to Lovecraft appeared.
Adams to Jefferson Oct. 12. 1812
My curiosity has been stimulated by an event of singular oddity. John Quincy Adams, at Berlin, purchased at an auction a volume containing three pamphlets bound together:
Woods Prospect, Wonder Working Providence of Zion's Savior in New England, and The New England Canaan, or New Canaan, containing an Abstract of New England, composed in three books.
[Adams quoting the third of those, by Thomas Morton]: "In the Year since the incarnation of Christ 1622, it was my Chance to be landed in the parts of New England, where I found two Sorts of People, the one Christians, the other Infidels; these I found most full of humanity and more friendly than the other: as shall hereafter be made apparent in due course, by their several Actions from time to time, after my Arrival among them."
In no part of the work has he [Morton] said any thing of Mr. Wollaston, his Commander in Chief, to whom he was only a second, in command of the Party. But it was of Wollaston I was most interested to enquire. I knew enough of Morton, and was therefore much disappointed perusing the Book.
The original Indian Name of the Spot possest by the Party was Passonagesset, but the People of the Company changed it to Mount Wollaston by which Name it has been called to this day. Morton, however, after the departure of his Leader for Virginia, chose to alter the Name, and call it Mare Mount from its Position near the Sea and commanding the prospect of Boston Harbour and Massachusetts Bay.
In his 132[nd] page He gives us a History of the Ceremonies instituted by him in honor of this important Nomination. Several Songs were composed to be sung. A Pine Tree, Eighty feet long, was erected with a pair of Bucks Horns nailed on the Top. On May Day this mighty May Pole was drawn to its appointed Place on the Summit of the Hill by the help of Savages males and females, with Sound of Guns, Drums, Pistols and other Instruments of Musick.
A Barrel of excellent Beer was brewed, and a Case of Bottles (of Brandy I suppose) with other good Cheer. English Men and Indians, Sannups and Squaws, danced and sang and reveled round the Maypole till Bacchus and Venus, I suppose, were satiated. The Separatists called it an Idol, the Calf of Horeb, Mount Dagon, threatening to make it a woeful mount and not a merry Mount.
It is whimsical that this Book, so long lost, should be brought to me, for this Hill is in my Farm. There are curious Things in it, about the Indians and the Country."
Jefferson replies with further information in a letter of Dec 28th, himself quoting from Nathaniel (not Thomas) Morton's New England's Memorial (1628):
[Wollaston removes to Virginia taking some of the plantation servants, clearly planning to take the rest at a later date. Morton tells the remaining servants they'd do better to remain with him, and they agree, ejecting Wollaston's lieutenant and taking over the plantation for themselves.]
"After this they fell to great licentiousness of life, in all prophaneness, and the said Morton became lord of misrule, and maintained (as it were) a school of Atheism, and after they had got some goods into their hands, and got much by trading with the Indians, they spent it as vainly, in quaffing and drinking both wine and strong liquors in great excess, (as some have reported,) ten pounds worth in a morning, setting up a May pole, drinking and dancing about like so many fairies, or furies rather, yea and worse practices, as if they had anew revived and celebrated the feast of the Roman goddess Flora, or the beastly practices of the mad Bacchanalians.
The said Morton likewise to show his poetry, composed sundry rhymes and verses, some tending to licentiousness, and others to the detraction and scandal of some persons' names, which he affixed to his idle or idol May-pole; they changed also the name of their place, and instead of calling it Mount Wollaston, they called it the Merry Mount, as if this jollity would have lasted always. But this continued not long, for shortly after that worthy gentleman Mr. John Endicott, who brought over a patent under the broad seal of England for the Government of the Massachusetts, visiting those parts, caused that Maypole to be cut down, and rebuked them for their prophanedness, and admonished them to look to it that they walked better; so the name was again changed and called Mount Dagon."
You can read the relevant portion of New England Canaan, where much the same story is told and Morton notes that "The setting up of this Maypole was a lamentable spectacle to the precise Separatists that lived at New Plimoth. They termed it an Idol; yea they called it the Calf of Horeb, and stood at defiance with the place, naming it Mount Dagon;"
Okay, so just to recap what we have here. A plantation in Massachusetts colony has a mini-uprising. Rejecting the customary solemn practices of the Separatist/Puritan Pilgrims about them, they start to party, adopt pagan rituals, and the assembled Englishmen and Native Americans carouse and behave licentiously amongst themselves. Their Pilgrim neighbors view them with distaste and place the name of Dagon upon them to indicate the level of iniquity there. Eventually, Merrymount was shut down by the authorities because they were supplying the Indians with guns, which could have repercussions if the Indians turned on the settlers in an uprising.
I see parallels with the story of Innsmouth, where the city undergoes an uprising and the conventional ministers are run out of town. Innsmouth residents adopt pagan rituals, "haowl on May-Eve," and take part in a certain amount of miscegenation, though the quantity of carousing is uncertain. Innsmouth's neighbors view the town with distaste, but the leaders of Innsmouth itself adopt the name Dagon for their new religious body. Eventually, the town is shut down by the authorities, since Innsmouth represented a future danger to mankind: "it ain't what them fish devils hez done, but what they're a-goin' to do!"
But is there any direct connection between Morton's Mount Dagon and Lovecraft's Innsmouth?
If we turn to HPL's letter of March 24, 1931 to Talman (SLIII, p.349):
"It may be remarked that their [the Pilgrims] action against Thomas Morton of Merrymount ... was not due really to puritanic bigotry but to genuine fear of the results of his giving rum and muskets to the Indians."
So Lovecraft was certainly aware of the events at Merrymount/Mount Dagon.
Furthermore, "The Shadow Over Innsmouth" was written "November-December 1931," less than a year after that letter.
I'm not going to say that this is proof, but I think the parallels are pretty strong, making it likely that some elements of this history germinated in HPL's mind into elements of the story.