For dual boot, Linux Mint and Windows 10, or for using external game storage devices, the RD2 steam files needed to be on an EXT4 formatted drive/partition to work. Steam/proton refused to load the game for me otherwise. The "Play" button would turn blue, then quickly back to Play.

I used Steam to move the game files, then had to verify the integrity of the game files and the Steam Runtime files. Steam needs all three Linux Runtime programs (scout, soldier, sniper) to work. Set forced compatibility, Proton Experimental.

Wine was installed, with the 32-bit libraries, over the course of trouble-shooting the above issue, so not sure if it has an effect on the final result.

To access the save files from both Windows and LM, this requires messing around with the "My Documents" folder. Open Windows and find a program to mount EXT4 in windows. I used Ext4Fsd. Use the program to mount the EXT4 partition/drive in windows. Then in file explorer, right click "My Documents" folder, go to "properties", then "location". Change the TARGET path to "(EXT4 partition/drive)/yourpath/steamapps/compatdata/1174180 (game#)/pfx/drive_c/users/steamuser/documents". I copied all my files to the new path, but copying just the game files might work. I was able to access my saves from LM after rebooting.

Will likely mess up your other uses of the Documents folder when using Windows.