You can specify acceptable shipping-destination countries in Seller Admin > Cart Shipping Settings; any country which you do not select in the list there will not appear in the "Ship to Country" list in the cart. Whichever country and zipcode the buyer selects in the cart is automatically passed to the checkout screen as the Shipping Address country/zipcode and is not alterable by the buyer there.
Note that the checkout screen also has a separate Billing Address section which comes first on the checkout page, where the buyer can select other countries/zips from selected/permissible destinations in the cart. This is to accommodate cases where e.g. a buyer abroad is buying a gift to be shipped to the recipient's US address. PayPal Pro will automatically perform the conversion from any foreign buyers' currencies to USD at current exchange rates.
We currently have no method to restrict shipping destinations more finely than the scope of Country, so you would want to explain your 48-state restriction clearly on your site, perhaps add a line of custom HTML to the cart screen itself using cart customization code:
We have received requests to add an setting to make phone number a required field, so that is already on the Development wishlist as a possible new feature in the future. Since our service is centralized and shared in common to all our Merchants, we can't simply just make that field required, as that would make it required for ALL our Merchants whether they want that or not. Most prefer to leave it optional in consideration of many buyers' strong aversion against providing their phone number whenever possible due to telemarketing, so making it required would tend to hurt conversion rates.
Our service automatically adds every buyer email to a "buyer group" list for each product they purchased, which you can use to send previous buyers a product update or newsletter. The sign-up checkbox gives buyers an opportunity to opt-out of having their email added to those lists, so they would not receive any unwanted emails (aka "spam"). As with phone numbers, many buyers appreciate having the opportunity to control if and how the email address they provide can be used, and we do not wish our service to become blacklisted as a source of spam, so we will not remove that checkbox; however, we can consider a wishlist request for a new feature allowing Merchants to disable adding any buyer emails to any product buyer-group lists at all (i.e., opting-out all buyers by default).