Greetings :)
I'm thrilled to share that I found a workaround to fix the download through Itch.
BEWARE: It might be easier to directly download the files as suggested by T-Hoodie himself (I think).
That being said, this is for people who like to have their stuff properly organised. As far as I can tell, Itch seems to only have trouble unpacking the RAR archive.
With a small nudge into the right direction, this can be corrected and the installation should finish without issue afterwards.
This means that the game will be listed in your library and can be launched through Itch.
In order to install the game through Itch, do as follows:
1. Initiate the installation as normal and wait for the error to appear.
2. Open Itch's downloads folder in the File Explorer / Finder. It should be in the same place as the selected install location.
3. Inside the downloads folder, find and open the temporary folder that was created for MtE.
-> NOTE: These temp folders have somewhat random names, so you might have to look through multiple folders to find the correct one.
For Windows:
4. Open the file "operate-context.json" with Notepad and look for the JSON key "Strategy". It should be approximately the 6th key.
-> If you have trouble finding the key, you can search the file for the word "Strategy" using Ctrl + F.
5. Change the value from 500 to 300 and save the file.
6. Retry the download in the Itch app (Downloads -> MtE -> View details... -> Try again).
For macOS (unfortunately a bit more tedious):
4. Open the folder "install-source" and manually extract the RAR archive inside that folder.
-> Since macOS can't naturally decompress RAR archives, you'll need to install an external unarchiver. I personally use Keka for that, but the choice is up to you.
5. Recompress the extracted folder as a ZIP archive (this can be done with the normal macOS compressor).
6. Delete the original RAR archive. Change the ZIP archive's file extension to RAR.
7. Go back to the root temp folder.
8. Open the file "operate-context.json" with TextEdit and look for the JSON key "Strategy". It should be approximately the 6th key.
-> If you have trouble finding the key, you can search the file for the word "Strategy" using Cmd + F.
9. Change the value from 500 to 100 and save the file.
10. Retry the download in the Itch app (Downloads -> MtE -> View details... -> Try again).
For those who are wondering:
Judged by the download log file ("operate-log.json"), it seems to be an issue with Itch's RAR unarchiver of choice:
https://github.com/itchio/boar/tree/master/rarextractor
I basically have no knowledge of Itch's backend and have looked into its workings for three hours or so just to find this specific workaround.
The Strategy key seems to influence which unarchiver mechanism is used. Finding the value 300 to fix the Windows installation was honestly just a lucky punch.
The macOS fix was the result of a bunch of educated guesses based on the Windows fix, trial and error, and the ever-changing installation error messages.
Throughout my research, I have discovered that some other Itch games suffer from this same issue.
While I have not tested this with any other games, I assume the workaround works universally.
Feel free to try this with other softwares too and if it works, please spread the word.
Thank you ^^