That blows, wish the car was here, as I'm sure we could all diagnose it, or give a second opinion. Perhaps it was caused from the previous oil starvation/no wastegate. Maybe it was just inevitable. Don't go beating yourself though, as I have found time and time again in this world, the worst problem often can turn into the smallest thing, and you can seriously beat yourself up over a mis-diagnosis very quickly if you arent careful. Take some time, and look over everything VERY carefully, and work from there. Find out if this truly is what happened. At this point obviously you have nothing to lose, and it could even turn into a very valuable learning experience.