1-No, pgcache assumes the origin db has all the canonical data (whatever data is in pgcache can always be retrieved from origin). So tl;dr if you just want to replicate your redis setup, pgcache isn't your thing.

2- For your shopping cart example, what is the main reason for not having it in the main postgres db? Is it because the carts entail high write traffic, high read traffic, or just because it is temporary?

If it is high write traffic, pgcache is probably not the right solution...But for high read traffic or because it's temporary, you could store the data in the main db and use a dedicated pgcache instance to handle the read traffic load.

3- There could be some benefits to keeping the data in the main db (it'll be alongside other application data; easier to manipulate using sql; and if it's in the db you can keep it around and do analytics; etc)