Yes, you can mount it on the luggage carrier if you select the variant with "mounting option: seat tube" and not "luggage carrier" for the product on Galaxus.

According to Hamax: "C1, C2 or C3 are indicated on the box of the seat. The designations can be considered as serial numbers. Although the Caress seats have different designations for each year, they are all the same. All seats are certified to EN14344 at all times." So it makes no difference. Caress equals Caress.