OK, this is one of those topics that long warped my brain but eventually I came to an explanation I liked, though it does require an unproven assumption of the Nature of Demonreach. But without it, there does not appear to be any need to involve Chronomancy at all, which ultimately is what foiled the plan by forewarning Harry&co.
I think Maeve's plan was to trigger the Banefire on Nov. 1st, but the Trans-Dimensional Nature of Demonreach required her to "toss the rock" from Oct.31.
We know Demonreach is a prison in Time as well as Space, so Here's the Assumption: Demonreach traps the inmates in a Perpetual Halloween. They are in "Stasis" and vulnerable, with the Banefire hanging over them (like a more literal Doom of Damocles, ironically), which is what it would take for the Banefire to actually kill them as Vadderung initially described. So if Maeve wanted to actually launch an attack on the Well itself (the magical construct) she could only reach it in Space&Time from the Island on the actual Day. However, then she is setting off a Nuke that she happens to be standing on, which is suicidal for all involved. This is why a Trans-temporal attack was required, it acted as a delay. But had she simply lobbed the attack from November 1st it could have hit the island but would not have "reached" the Well itself.
Support:
-Per Bob/Alfred the Well is a Prison in Time as well as Space, and maybe even higher dimensions (ie parallel worlds?)
-The British Inmate raves about the inmates being in "Stasis" and how the presence of the Warden "Buggers that up entirely." This could be taken to mean that the inmates only experience the passage of time in the direct presence of the Warden. Functionally it would make sense for the Well to go out of it's way to facilitate interaction since it's the Warden's responsibility to Determine if they should ever be Released.
-Vadderung indicated that the Inmates would not survive the Banefire. But Maeve (and Harry) were talking like they assumed the inmate's would survive.
-The Chronomantic nature of the attack itself was a huge tactical liability without any other (obvious) benefit.
Counter-argument: The main counter to this idea is that holding the inmates in perpetual Halloween could be MORE dangerous rather than less. For the same reason that some Immortals go out and Play on Halloween while others hide, they are more vulnerable on Halloween but it is also the only day they are capable of Change enough to gain new Power. This is not so much an argument for why it would not work as it is one side of in-world the Risk Assessment that the Well's designer(s) likely had.