Just to put an end to any doubt, or worrying about missing out on something, it is absolutely NOT possible to have sex with Minami currently through any type of cheat or modification, the code/dialogue for it simply doesn't exist. However, there are two things that are currently unreachable (unknown why dev did this), that can be done with code modifications.
I would provide patches for this, but it doesn't look like it's possible without completely rebuilding the archive.rpa file. Let me know if anyone is particularly interested in how to make this happen.
You don't necessarily need to rebuild archive.rpa, you can extract and decrypt the specific .rpy files, modify them to your heart's desire (for example, for the sex scenes each character has a file called fuck.rpy), and place them in their desired paths on the disk, respecting the path they had in the archive.rpa file (eg: game\ch\harmony\fuck.rpy), and I think Ren'Py will prioritize the modified .rpy file to the one in the archive.
To decrypt all the .rpyc files (complied versions that are in the archive.rpa) into .rpy files (readable decomplied text-based versions), you just need to google "un.rpyc" - place that file into the "game" folder, and on next start all the .rpy files will be decompiled and written to disk to their specific locations.
Just make sure to move them somewhere else on disk after they've been extracted, and remove un.rpyc, or the script will do that all over again. Also, if you keep them where they are, the game will crash.
The clean game directory structure should only contain the folders "cache", "saves" and "yaml", and the files "archive.rpa" and "script_version.txt".
Good luck!