Hi Valantis,
3. Sellers can set up their own shipping rules. i.e, Depending on Weight, Size, Region etc. and it will be applied to the product.
7. Yes, It is just email of users.
8. There was a messaging system in place but it has been disabled for some reasons. For now, Buyers can contact admins via email.
Regarding your other questions,
1. Let's say Paypal charges 2% on each other + 0.30$ per order. You can either take the cost on your own or transfer it to buyer using these options. The amount would be charged extra.
2. It's in Global configuration > Shop Setting. Select 'Allow Guest Checkout' to No.
3. Shipping is applied to products via Shipping rules.
4. Don't create any promotion code and hide it via CSS.