Just to give it a push.
My brother and I do have exactly the same problem. We didn't try all the things the OP did, but it seems it wouldn't help anyway.
The game did run very good until we reached Lower City BG. In all the hours before we only had one or two game or synch crashes that affected both of us.
But now I (as the guest player) get stuck in the loading screen (loading status number showing 100%, no loading art, only black, loading circle not moving) when joining his game. It mostly works when task-killing the game and start again, but when I emerge from the sewers or a cellar, I get stuck again or the game loads and I see a little bit around me, but my avatar and party member, almost all textures and UI is missing. The game gets stuck very soon after.
This is no fun.
We will try to go to camp when saving and sending me out first and try to pursue only the main quest, to bring the run to the end. But if this doesn't work reliable, we will have to stop the run, which would be a real shame, because we had so much fun until now and the game is so great. ;_;
