Cannot Join Campaign Games (Multiplayer) Nor can my friend join my hosted game.

I've removed all mods, deleted the script folder, everything short of re-downloading the game.

I mostly want to play co-op with my friend.

Any fix?

I crash to desktop when joining other peoples games and his says incompatible versions, even tho we both removed all mods.

We've re-validated thru steam a handful of times after deleting stuff as well.

I am currently re-downloading the game after deleteing it from steam, which I doubt will fix but I am so angry Its the only thing I could think of.

I did forward the ports for steam+shogun 2

It sounds like one of you has a patched S2 and the other doesn't. Make sure you have both downloaded the latest updates for Shogun 2.
Aside from that, I think you are on the right track with a re-install.