As copyright holders it is up to Bethesda as to what form redistribution of their copyrighted materials is acceptable either in full or in part.
They will likely allow redistribution of their materials if it benefits them in some way, eg if you include retextured models in your mod they will probably allow or even encourage it as additional content increases the shelf life of their games. If redistribution of their copyrighted materials contstitutes piracy and/or likely to harm their business then they will likely intervene and put a stop to it. Your mod sounds like the former case of being likely to increase the shelf life of MW which is not the same as unlicensed selling of their stuff nor harm their business and therefore likely that Bethesda would not object. IMHO
Exactly.
There are numerous mods that include retextured meshes, and if Bethesda didn't like it, they would have done something about it by now I'm sure.
In-particular the models that you actually DID retexture (even if the new textures are also from the vanilla game), would qualify as a significant enough modification that it should be allowable.
The fact of the matter is that people need to own MW anyway to use these files (unless you're some weird sort of person who downloads MW meshes just to look at them in NIFSkope or something). It WOULD be illegal and disallowed if you were porting these unused meshes to Oblivion or Fallout 3, but as it stands you are not, and this should be allowed.
If you heard your information from a moderator/administrator of TESNexus, then it is merely against
their policy. PES may be more lenient. Right now though it looks like it's still up on Nexus and if it's just some random troll telling you it's not allowed, ignore him/her.
Of course, if you'd rather just play it absolutely safe, you can just go ahead and make a list and maybe include some recommended textures to use as starting points when the textures do not exist.
Edit: Oh, looks like you may have already made the change and uploaded it to Nexus, and that's why it's there...
Also, I should mention I think someone may have done something like this before-I can't quite remember...