I would def go with Shub as the starter old one in attendance, reason mother of 1,000 young seems to go well with fertility rites. Leave the other gods for sequels or later on. You could totally stretch this out to be a multiple book series. 7 is a good number to start with, enough to pack insane horror acts, with the main getting the blunt of the story. As for the town have them emerge and disappear. Like in between realities/dimensions. To explain the shift, star alignments, or certain town olk can't cross areas due to being held to the locale either by magick or ward. Another thing could be there are many people there in town but say only the Shub people are relevant to the story. Others who are there for say Azathoth aren't interacted with in this story. It opens up more later if you wish to delve into it being a lovecraft hub society.
A good movie reference you could check out for ideas, though not Lovecraftian, would be:
The City of the Dead (Christopher Lee)
I wouldn't pare it down, instead I would outline the events and divide into sections, brainstorm the events.
I would break it down as such:
1: Status Quo of town: Describing the area in details
2: The ritual: Introduction to the occultists
3: The arrival: College students arrive and the events introducing them as characters
4: The Plot: Occultists take note and scheme plans
5: The interaction: Occult meets College and start the plans
6:The taking: character gets taken friends unaware, breaks friends apart from each other
7:The breed lair: What happens to taken one
8:The farm: Whoever gets taken here in this section, and what happens. delay any death sequence for later build the reader's imagination while rest of story continues.
9: The alarm: Friends become suspicious their other friends are missing, go in search of, with or without occultists "help"
10:Kill off your darlings: Use this section to kill off loose characters
11:The Farm horror: Outcome of the farm, adding more to it if need be from group.kill off the first one with their friends there to witness it, and get an idea about their fate.
12: The taken gets impregnated: Describing the events and the outcome and how it happens.
13: Friends smarten up: Realizes whats going on and comes up with plan to settle the plot.
14: The climax: Either the end of the antagonist or they get away.
15:Taken one found and retrieved: get off the island with survivors. Either reveal at end in subtle way she's pregnant with an old one's kid or save that for the first part of story 2.
Just how I would do that story.