It would be possible to use Payments Standard and rely on the IPN post to get the details of the order back to your site. On your IPN page you can repopulate the cart from the post, then you can do any of the regular cart stuff like storing the order summary and details in your db. To make it work with the admin backend though you will need to model your store order summary and store order details server behaviors after the ones on the default PowerStore pages. You can open up the confirm page and take a look at the server behaviors to see how they are setup.
For forcing the user to login you will need to do something custom since you will not be able to use the PowerStore code for this. The idea is that when the user logs in their id will be held in a session variable called UserID. You can check for this session variable having a value, if it does not then the user is not logged in and you should display a login link rather than the Pay with PayPal button.
For the shipping price this should be included as a charge in the cart and passed to paypal as the shipping or handing amount. You can use the custom field in your checkout to store the type of shipping requested. It will then be a part of the order that you can store in the db.
You mentioned that you have some other threads on this topic out there. You should try to keep this in a single thread to avoid confusion and duplicate responses. You can add in links to the other thread for reference if you need to but it should all be in a single thread.