This exact thing has happened to me personally and others I know and it's always been on flights that had been booked many months earlier.. for no apparent reason the seat is switched from the upper deck to the lower deck. An annoying problem to be sure and the only advice I have is to just keep checking back and snag open seats if they become available. The idea of compensation is a bit ridiculous to me especially if you can still get 2 seats together in C.

As far as flying backwards.. have you ever done it before? I have.. and it's really no big deal. The only time you even notice a difference is on take off and landing.. a very short period of time. While in the air there is no difference so motion sickness should not really be a concern. It isn't the same as riding backwards in a train where you feel constant motion with starting and stopping and can visually see the world going by backwards out the window.