Zargerth

Mightyena is a good choice, especially with Moxie ability. Then there's Linoone, but you'd have to level Zigzagoon all the way up to 20. Goldeen with Lightningrod as its ability is a decent pick as well. You could also grab yourself a Trubbish mainly for Toxic Spikes.

It's been a long time since I played Rejuv, but I'd go with Mr. Mime, because it learns Reflect and Light Screen early on, you can also teach it Flash (hint: Flash can change some Fields to your advantage) and for what it's worth it can get Filter as its ability. None of Mr. Mime's abilities are useless, but if you go with Technician, I believe Confusion is stronger (50BP x 1,5) than Psybeam (65BP) so be wary of that. Soundproof also has its uses, albeit more situational.

Try loading a backup save. Chances are your save file is corrupted.

Meowth can be found in the caves, so make sure you're there and not in the jungle. I'm fairly certain he asks for the same mons, but I'm not sure if the order of the mons in that list can vary. He asked me to find Bibarel as well. I think after that it was one more mon which I can't remember, and then a Pidove.

The higher your IVs, the better. While you don't need to completely obsess over having high IVs, they're nice to have and might even come in clutch.

From what I understand, the battle theme is same (at least between E4 and E5). Am I missing something or did you mean Team Crescent?

I'm not very tech savvy, so I can't say for sure what's going on, but my past experience with weird errors leads me to believe your save file might be corrupted. Have you tried loading a backup, if you have them enabled?

You can buy those from the evolution item store in Central Cellia. It's an area dedicated for EV training, where each of the smaller caves correspond to a stat. Cavern of Agility being the place for Speed, Cavern of Mental Strength for Sp. Attack and so on. It's recommended that you buy the Power Items from your manor shop before EV training.

I could be wrong, but I remember seeing it being sold at the Jinx Guild credit shop. This would mean it's available only for a limited amount of time.

Have you found the sewer entrance located in an alleyway south, almost opposite of Fight Club? Walk past the hiker and you should see the entrance.

You have something like this, correct? Or have you already extracted it? If not, you need a tool like WinRAR or 7zip. Once you have either one installed, you just, well, extract the Deso folder.

So you have neither of these? You've downloaded the folder, but have you done anything else?

I think you can drop Animations, as those refer to moves used in battles I believe. Of course, if you're planning to use those as well, then in that case leave the credits as they are. You're welcome, although there is no need to thank me for any of the sprites, for I have nothing to do with them.

Run Game.exe or Game-z.exe, either one should work.

Yes, there should be one available, but the thing is I don't remember where I got it. Besides that one, you can buy them later with Credits. No, as far as I know you cannot open it. However, the house at the very top (where you met Tristan) can be opened.