Since this could use a conclusion:
Me and @_fjrxx managed to get their save file working again! Hurray
So I wanted to try and summarise some of the things we tried and discussed, so that others may also benefit from it. So here goes
1. Make sure your save file actually ends up in the folder. This sounds simple, but I noticed that getting confused about what is actually in your folder can also be quite simple. With this it might help if I take the time to explain what a save file looks like
You see, an active save file, the file the game will look for and read, is called Game.rxdata. If you have more that one save file, the other active save file will be called Game_2.rxdata (or Game_3.rxdata, etc.). Then what are these other things in my folder, these Game - number - more info.rxdata things? Well, do you know how the game asks, when you save, 'There already is a save file, are you sure you want to overwrite it?' (yes I know you've been skipping over this text since you were 5, it still asks though). That is what those other files are, your previous, overwritten saves!
Hopefully this helps with recognising what is actually in your folder.
2. Make sure your save file has the correct name. Like I said, the game is going to read active save files, and it knows what an active file is because it looks for their exact names. That means your file has to be called Game.rxdata or Game_2.rxdata (or etc.).
Now, your mac isn't going to like it if you try to put a Game.rxdata into that folder when there is already a Game.rxdata there. Same type of file with the same name in the same folder is a no no. You can just rename one of them to Game_2.rxdata (or etc.), so you can keep them both without any issues. Additionally, I would like to add that the name should be exactly Game.rxdata (or Game_2.rxdata, or etc.). Naming it 'Game.rxdata old save file' or '(copy) Game.rxdata' won't work, because the game is not going to search for those files.
3. Now the next issue we ran into is one we actually haven't managed to fix. You see, when @_fjrxx moved their save file to the folder again and had renamed it so the game should kn ow to read and run it, the file changed into a txt file, now looking like this (with TXT)
rather than a blanc paper
Since the game would still not start up their save, we assumed the unexpected txt thing was the issue, the game probably didn't know what to do what that type of file. We however did not manage to figure out why or how it turned into a txt file, and I also couldn't come up with any ideas on how to turn it back into something the game could work with. So yeah, the txt file issue remains a mystery for now, and if anyone has a clue what could have happened, please let us know
BUT! combining some of the things we mentioned earlier, we also found a way around it!
4. You see, the first thing we discussed was active save files and overwritten save files. The active save file has turned into a txt file and we didn't know what to do with that. But @_fjrxx had send a screenshot at some point that showed they had a lot of overwritten save files in their Wine files. Overwritten save files still contain all the information from that save. They went to find their most recent overwritten save file, so we could use that because it would hopefully only mean losing a few minutes of gameplay, and then all there is left to do is turn the overwritten save file into an active save file. How do we do that? Easy, just rename it. Call it Game.rxdata (or Game_2.rxdata, etc). The game looks for the correct file name, that is the save it will load and run and that you can play with.
So they did that, found a previous overwritten save file, renamed it, and again moved it to the new Pokemon Reborn folder. The file did not change into a txt file this time (which, yay, but also, why are you not being consistent!?) and @_fjrxx could start up their old familiar save file and say hi to their loyal Pokemon squad. The end.
Anyhoo, if anyone recognises some of the issues mentioned here, hopefully this might give you some insight and help. If you're not sure what to do or have a different problem, leave a comment or shoot me a private message, and then I'll think along with you for any solutions.