Thank you Gap, I appreciate your details, I will take a look in the names.cfg files, also your point 2 can be checked easily by modifying an entry date in a single mission to see if the game crashes or instead if the ship doens't spawn (which I remember was the case). An important trait in SH4 & SH5 is that single missions will *always* crash the game after 72 hours or so gameplay chrono time (I know this from SH5, but maybe not the case with SH4.)
As a last ressort I will have to go through all the unit files in Sea folder and the campaign mission files to see if these units are correctly referenced like in the names.cfg you mention, this is cumbersome but possible somehow. Normally I use to delete from a mod the Campaigns, the Sea, and Roster folders if I want to revert to stock campaign for testing and it 'generally' works but there are exceptions.
Have you ever seen or heard that it's possible to make any unit go along with a player's submarine in a mission/campaign ? I mean for example that a destroyer would always follow the waypoints of the submarine like in a defined convoy group? Of course such a 'player group' must be setup in a mission file.
Thanks very much
XS