1) Customized theme looks nice 2) On the Register / Log in page, it still says " By creating an account at nopCommerce", I guess that needs to be changed by your store name.
i didnt need to make any of these kind of changes to my website as i was quite happy with the existing functionality.
if it were me, i would be looking at the CheckoutShippingAddress.aspx page and finding the module specific for retreiving or saving the shipping address.
in the above module, you should be able to find which table in the database holds that information.
add the new field in the database, then update the code for the website/data access layer where required.
not a trivial task if you dont have much experience with asp and c# but it is what is required.