Your shoulderbelt does lock, just pull it all the way out, slowly, and it will ratchet back and hold tight (the vehicle manual will talk about that in the car seat section. You can check the Key Fit manual to find out what they advise, unless you saw the instructor read that right out of the manual, they may not be entirely correct, all seats have different rules about locking the belts when there are lockoffs on the base).
I might just put it in an outboard (window) seat with the LATCH, as the Accord LATCHes are away from the doors by quite a few inches, and the install would be a lot easier and more solid (no matter how good the seat is, no seat is compatible with ALL seating positions, it's just not possible, and also make sure your lockoff isn't bent or damaged, that does happen from time to time and Chicco replaces the base).