No, no need to install OpMonsun for the IJN subs to work. There really isn't a whole lot to do in just that JyunsenB mod, and you might be interested in Cybermat47's mod
Japanese Career, which works with keltos_01's subs. It is basically the German side of the game, with some files changed to reflect the IJN subs use. However, get the JyunsenB working first, since it is a pre-req for Cybermat47's mod.
Now, your install folder is good. As for your "Save" folder, what you have there is what MS Win10 is intercepting after Windows most recent set of updates, designed to force folks to use their One Drive, like Apple does to folks with their devices and the iCloud. But see if there isn't also data in a "C:\Users \UserName \Documents \SH4" folder. Several players recently have been reporting it empty though, and that Windows 10 is now exclusively using the %$#@*& idiotic One Drive... in which case, you delete a file from the hard drive, and Windows instinctively knows you really didn't want to do that, and restores what was there from the "cloud"... so check that One Drive folder again also... I have one computer doing that, and I still have not figured-out how to bend it to ~MY~ will, and not Billy Boy's...
Now, did the game run fine previously? Prior to attempting to install the JyunsenB mod? If you back the mod out, does regular SH4 function once again?