No one said anything about magic resist. Thornmail is just bad. You could have done better with a Sunfire Cape.
Yep, although they are AD, buying Randuins against them would be like buying RoA on Katarina because it gives AP. It's a good items though, but you could have still dropped Thornmail in favor of something else.
Also, no one had Banshee's veil. One good knockup from Cho probably made that game, which could have been prevented by the item. IDK, just saying, there are better option to be had.
Again, I ask why didn't you help your team? If Cho was top so 4v4, if you went there it would have been a 5v4 in your favor, gg easy win. Seriously man... You already said you and Cho couldn't kill each other so top was practically a perma stalemate, and nothing to be gain from it.