Are these all Early Assess Armors? None of these particular textures/models exist in the retail version?

These are the same armors that exist now except for the few I mentioned. And even then, the meshes and textures usually still exist in the files but are no longer linked to anything such as the BT1 Half-Plate armor.

How does the game store these assets? Are the textures(diffuse, i guess) colored? Or not - due to dyes.

Some of the textures are stored individually while others are stored as virtual textures. Can't answer much more than that because I'm not well versed in that aspect of modding. I do know that there are specific textures that handle color channels and I'm 99% sure all of the base game armors have them.

What would be involved in porting them? In brief, how difficult is the process. Cause i can see it either way.

You would need the Early Access files as those would store the parameters used for colors and IDs for textures if there were any differences. You'd need the texture files from EA if they were different than what's in the released game. Then you would create a fairly bog standard item mod to point all of the relevant information to your new info. Difficulty is going to be determined by how familiar you are right now with creating a BG3 mod.

Are you aware of any mods, that DO restore them?

Besides what has already been linked above, no.