Well done tracking down the file.
Quote: " I didn't find any settings in AppGameKit that would let me change this folder"
Did you try any of the AppGameKit commands like SetRawWritePath or SetFolder?
Quote: "The sad news here is that "myfile.txt" would not open since the Mint text editor says it can't detect the encoding."
Whether you can open the file in a text editor will depend on what you have written into the file. If you want to create files containing human readable text, you need to use the WriteLine command. All the other WriteX commands write the data in a binary format rather than a text character encoding.