It gets corrupted by PKHeX, NOT the save manager. I'm using Checkpoint just for this, but if I load the save on PKHeX and do even one thing, it gets corrupted!:/
The legendaries are in the Altar of the Moone in Poni Island. Just get there directly with Charizard and go through the ultra wormhole. Little advice though, if you end up going after Kyogre, you REALLY need something like a Lapras! I'm at Kyogre and my team gets wiped at level 90 because I'm trying to catch it while it spams Calm Minds and Origin Pulses!
I only changed the "date met" for all mons, the EVs for the legendaries (EV training is a pain in this game), date of adventure start and basically all dates because I noticed RTChanger set my year to 2030! Besides that, I just gave myself more Pokeballs (the Johto ones and Masterballs), Big Nuggets (for money), Rare Candies and Bottle Caps. That's all, no major edits, didn't even bother genning anything yet, but it gets corrupted if I do ANYTHING on the save, even just nicknaming something corrupts the save, even with the Pokedex setting unchecked since for whatever reason it goes automatically from unchecked to checked and that's how it gets corrupted... Should I try a different version? Also, you get only one of each legendary, but infinite amount of the UBs that are inside the wormholes. Unchecking the flags in PKHeX allows to find legends again though...
The UBs are in the ultra wormhole, so you get them from there, no missions from the looks of it. After beating Giovanni, there's not much to do, just do Battle Tree, Battle Agency or fill up the dex.
I feel you, that mini-game gets old VERY fast...
Magikarp Splash is a hidden move that's never added to the movepool, but it's not worth it, it looks like it gives lots of points but it gives VERY little compared to even the bronze ones. Keep spamming the highest level ones you got, get first place, unlock gold+ ones and then watch your score go to 100+K while you make more BP than even the Battle Tree!
Gamefreak office on second island, next to that place where the prof's wife works. You talk to the guy in the white Aether scientist costume to change controls.
If you got a Poison Touch Grimer, just use Crunch or something and if you get a poison on it along with the damage, it won't last long
Surprisingly, I haven't. Don't know why they made them so rare in this game!:/
PS: For anyone wondering, the games are NOT shorter than S/M, just have less of a chore things to do. They don't force you to go to some random area just to get UBs just to pad out the time, but Rainbow Rocket takes longer than the Looker UB missions, so post game isn't shorter. Also, for main story, it's longer by about 2-3 hours, just less tiring so you don't really notice it.