I was reading on the reddit the other day (have to keep up to date on new drops for the Home dex*) that people were doubting that specific infographic being entirely true. Something like the breakdown of it was based on a separate, much more accurate leaker, with gaps filled in by educated guesses based on other patterns.
So while Smoliv for the Harvest festival, Tadbulb for the Festival of Lights, the Shiny Guzzlord and I believe Timbur community day are all "locked in", they originated from a different leaker (the other information has been 100% accurate). So it's possible we don't get Greavard but do get someone else, like Honedge or Mimikyu.
*No I'm not looking forward to praying for the Skiddo in the upcoming regional Go Fests why do you ask.
So far the only thing that's in doubt is Shadow Lugia in raids on the 7th/8th, but that'll be announced shortly if it is happening so not long to wait.
Otherwise as you say it's pretty much educated guesses - Regigigas is a reasonable guess for Giovanni, and Skorupi is as likely as anything for an incense day. I did hear a rumour about Carvanha a while back but that's unlikely imo.
Very much hoping the Mega Garchomp leak is true though.
Edit: Actually while I'm here, how many megas are left? *checks Bulbapedia* Mewtwo x2, Heracross, Mawile, Camerupt, Sharpedo, Metagross, Gallade, Garchomp, Lucario, and Audino...
Really surprised not to have seen Audino or Mawile yet, and surprised they didn't release Mega Camerupt and Sharpedo either during or in the leadup to the Hoenn tour. Though obviously they'd have been overshadowed by Groudon/Kyogre... which is still true now tbh. Kind of dropped the ball there, Niantic...
The remaining ones are all going to be very hyped so I get why they've been kept until last. I'm guessing Mewtwo is quite likely to be featured in the next Go Fest.
Gallade and Heracross are tricky: Gallade is notably the only split-evolution mega so they're probably trying to figure out a way to make that work. Just randomise the energy drop from buddies, I suppose? And Heracross is a regional, though that didn't stop them doing a raid day for Kangaskhan (can we have another one of those soon pls) and it was available freely recently to everyone in Go Fest. At a guess I'd say it probably gets a raid day around March next year as part of a Bug- or Fighting-themed event.
Similarly, the fact that we had a hatch day for Riolu recently makes me think that Mega Lucario isn't a million miles off. They might try and milk it one last time before they let Lucario be shiny in the wild (not that I expect it to be a wild spawn, but you get me. Isn't it a reward for one of the level tasks?)
Was there a leak for the GO Halloween event? I was just guessing Greavard because it was the worst ghost they could release, so it seemed like just the thing Niantic would go for...
The only definitive thing (beyond the release of Greavard and shiny Phantump/Zorua) is that the second half of Halloween will feature a Rocket takeover. Presumably there'll be 2x candy during Halloween because that's the traditional bonus, and I imagine Galarian Yamask will feature in some capacity since it was last year's rare new species. It'll probably be in rarer/harder field research.
Of course I think I speak for everyone when I say what I really want is Spiritomb freely available in some capacity. Seriously, even if it's in "win 3 raids" tasks I'll take that.
