CA doesn't list any limitations on which flights you can check in that I could find Interesting is that CA list OLCI available as much as 36 h to 3 h before departure, while LH has only 23 h, which should allow me in August to checkin for the first 3 flights of a Milage Run.

PS.: when I booked my tickets at the CA webpage I could choose C seats, for those legs operated with CA metall. Are all of your legs with CA metall? If not you have to get the booking reference for the airline your flying on to reserve seats.