As long as nothing in your game is calling on the ship to spawn, I don't think just having a folder in data\Sea by that name with nothing but .texture files would be a problem.
The first step would be to do a search of your Roster folder for T15B.cfg, if that file is not present in any nation's roster then it will not spawn under any flag so the ship should not appear.
AFAIK the MFM ships are not individually scripted into any campaign files so there shouldn't be an issue there (someone please correct me if I'm wrong).
It could be that the validator popped up an error because there is a data\Sea folder for the T15B class of ship, but the folder doesn't contain all the files necessary for that ship to function properly should the game try to render it, which I don't think it would do anyway if that class is not in some nation's roster and/or specifically scripted into a campaign file.
I'm just guessing here based on experience so if someone knows anything different, please chime in.
