From the sounds of it, the new chain stretched some. With my new Trek, after some miles were put on it, it started shifting all on it's own... like you said your bike did. The chain was just stretched out and the shop had to fine tune it.
Although, if I were you... I would take it to someone else and get a second check done on it. It's not worth the risk.
Otherwise, it's not the week for this kind of stress... you are right. Luckily though, you don't have to train as much... right?
Good luck D... I am sure everything will fall into perfect place on Sunday.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"