I used to work in Lenovo Tech Support few years back.

Cosmetic defect indeed need to be attend by SC instead of Reseller. If hardware defect can exchange 1 to 1. There is also a funny situation about deal pixel , need to have 5 dots and above
Communications between reseller and Lenovo kinda P&C , unless user demand it then Reseller could inform Lenovo to send a formal email.

For warranty part, not sure if they change antything , as long you have proof of purchase , doesnt matter you register or not it still follow according to receipt as long the S/N and model matches.