I believe I have fixed the problem with SMS Schleswig-Holstein
here. The cause was identical IDs shared with a Greek warship. Thanks to s7rikeback for the help!
Unfortunately, it seems I'll be abandoning the mod anyway as I also took a look inside the mission editor and confirmed that the campaign files are indeed missing plenty of stuff from the 'GWX - Merged Campaign'.
EDIT: s7rikeback has just finished crafting a very thoroughly corrected version for us
here!
Regards