I've been gone for a long time as college and work are taking most of my time (and most of my spare time is now spent at keeping up with seasonal animes), but happened to be lurking here last night and saw jojo's request regarding Poké Pelago, so decided to try it out, as I had plenty of research material -- by which I mean hundreds of save backups on a recycle bin that hasn't been emptied in years.
This is what I found out:
The encounter with Mohn after the fire trial is controlled by event constant 470. When he's ready to show up, its value is 18000, and after he talks to you becomes 18200. However, as I expected beforehand, changing that alone won't do any good, as the menu will lack the Pelago icon even if the event is flagged as done.
After a long and extremely tedious search in HxD, found the menu data. Then replaced the values between offsets 4600-462F with this:
That enabled a functional Pelago icon on the menu, so I tried it on an even earlier save (before being dragged to the Trainers' School) and worked just fine, got access to Poké Pelago without having even completed the first trial.
No Charizard ride flag needs to be manipulated.
So, got these and dumped'em:
041 - Zubat - 7D4CC628B493.pk7
086 - Seel - BBC8B9E66C4A.pk7
278 - Wingull - 61E2C8C68B7D.pk7
731 - Pikipek - 4556223AD842.pk7
Lacking Cutiefly
Also, these three stayed, so here, in case you want the things to have actual trainer data:
086 - Seel - B5D8B9E66C4A.pk7
278 - Wingull - E954E6BE24C2.pk7
731 - Pikipek - BA51A9B71124.pk7
Seel is the same as the dumped one, but Wingull and Pikipek are different. Also, met dates might seem impossible as of today, but that's just cuz I changed my device's date to accelerate the process.
*All of this applies solely to USUM. Haven't checked whether SM is any different or not.