(The delightful Aoife McLysaght courtesy the 2018 Royal Institution Christmas Lectures.)
You never need to ask nicely to get me to make graphs. :D
First, though, I have to offer up a few suggestions for improving the timeline:
-Which was the last mission posted before The Takedown? You have it as 15, assuming that the A/N was added when it was rehosted. That makes a certain amount of sense: one of the things FF.net has always despised is 'interactive stories', and TOS15 is the mission where a bunch of reviewers and fans show up at the barbeque. It may have been the metaphorical straw that brought the hammer down.
But... that author's note really reads more like "hey look, we wrote another mission!". The biggest clue is in Jay's comment: I ceased to be bitter about this some time ago, and just view it as INCENTIVE to get our mirror site up. (Emphasis added.) If this was a rehost of the last posted mission, would she have made it sound so long? I'm pretty sure there were reviews saying P@L was hosting stuff within a couple of days. For example, Tina on this page says it on the very next day!
-When was TOS15 "The Dark Elf" posted? I'm still not sure exactly what happened on March 12 2003, but it wasn't the founding of the Board - that was up before December 2002. Best guess is that P@L moved her site, or had a crash and relaunched it. (In fact, the Tina review above includes a really weird partial link; if it's to P@L's hostage, then she definitely moved it later.)
Neither of these change the overall picture too much, and that picture is surprisingly simple!
Jay and Acacia worked at a constant rate until they were taken down. Then, on average, they shifted to a new, lower constant rate for the second half of the series. The break is even more apparent when you break the graph in two:
You can see from the equations that their rate of publication pretty much quartered when they were taken down. Before the Takedown, they were posting a mission every 11 days or so; afterwards, they dropped to every ~45 days.
The big change from that is missions 17-19, all posted in August 2002. As Neshomeh pointed out, this is the school holidays - and in missions 20 and 21, Jay mentions that she's been busy with school. It's so delightful when things come together like that.
... and I'm still finding new information. I've tracked down the original Oddlots link! It confirms the following latest dates:
-TOS17: before August 3rd.
--In fact, TOS17: June 29th, 2002.
-TOS20: 9 Oct confirmed.
-The Board: linked by December 11 2002.
-TOS21: 02/12/27, A christmas present from our Original Assassins.
All of which smooths out the graph even more:
I'm also trying to hunt down Arwen Lune's website, mentioned by Architeuthis in the Clan of the Cactus reviews. She's on FFn and fictionpress, but sadly the latter has stripped out the link to her personal site. She may be the same Arwen Lune as is on the L-Space web - she's certainly active in fandom at the right time - which would make this her website (and this, though there's not a lot there). Indeed, CuteAndFluffy has a bunch of LotR stuff on its links page - but not the PPC, or any fanfic, or any forum.
FAKEEDIT: Definitely the same person: her FF.net profile links to her fanfic LJ community, which links to her personal LJ, which mentions CuteAndFluffy. There is only one Arwen Lune, it seems.
(It looks like the Hidden Archives also mentioned by Architeuthis never hosted the PPC, though they do have Architeuthis' Intelligence briefs if we need a date for those...)