Express Checkout is the most robust API to use. The "return" to your site is a good thing, to give the customer the ability to click a final "Place Order" button. But you can also have this triggered automatically on return -- just run DoExpressCheckout without user interaction and display a thank you / confirmation message. If you are going to run DoEC w/o interaction, your initial redirect to PayPal.com should have &useraction=commit appended to the URL. Then, the final button at PayPal.com will say "Pay Now" instead of the default "Continue"
To allow guest checkout with EC, pass SOLUTIONTYPE=Sole in the initial SetEC call. Note that when customers load a PayPal.com checkout page, it checks their browser cookies to see if they have a PayPal email address stored. If they have one stored in the browser, the top "Log in to PayPal" section will be expanded. If they do not have an address stored -- generally meaning this is a brand new computer or they've never used a PayPal account to check out on this one before -- then the bottom guest checkout section will be expanded.
To not require an address, pass NOSHIPPING=1.