1. Technically how they managed - not sure how - all are "maybe".
2. Yes - Smart card contains the receiver number and verifies accordingly.
3. No - this is not the way.
4. No - not possible to watch through DVB cards.
Evenif you change the decoder (the same DISHTV provided KAON receiver to another DISHTV KAON receiver), will not work.
Discussed several time here