Heyho, I just had a quick look on the Excel sheet in the constribution zip.
Just one thing I noticed, the sheet isn't quite consistent in what it lists. Some events that are missing are listed, but others like the aforementioned Pikachu, the different TIDs for the Mega Campaign events, or Australian Hoopa (ID: 10235) are not (maybe more; I have not checked it really, these were just the ones I noticed).
I'd also suggest to include the Pokemon Link Events, where Pokemon Bank Meganium, Typhlosion and Feraligatr seem to be missing.
Some kind of overview in this thread or in the sheet which events the collection does NOT include yet would be awesome.
Another thing I noticed: There are "?"s for Electabuzz without ribbon. I believe it is safe to assume that those don't exist. I've been quite active in the event collection scene at this time, and have not seen a single Electabuzz without despite actively searching for one. This was before the time save editing other than powersaves was possible. As there have not been any of those, it seems to be very likely that the Magmar without ribbon exists just due to some mistake on Nintendo's end.
As I haven't been following the scene for a while and am too busy to get up to date for now: In case you are already able to decrypt powersaves-backups let me know, I could contribute some of the missing events, such as different versions of Magmar with/without ribbon (I was able to get quite some codes at that time to redeem myself) and the Italian Arash Mamoswine (picked up at German VGC on Italian language).
While I can juggle stuff through my japanese copies with CyberGadget, I have no way to access the save of my european games directly, so unfortunately I cannot extract wc6s.
Edit: Forget about Hoopa, I just have overseen it in the list.