Hi,
1. No, PayPal Express checkout is a payment plugin available for all the editions of HikaShop, even the Starter one.
2. Because when the payment notification is received by HikaShop (PayPal sends that to your website in order to tell HikaShop that a payment was received for an order), HikaShop has to check that the amount paid by the customer correspond to the amount of the order. If there is a difference, it's potentially that the customer changed the amount to be paid and you don't want to validate the order (it would be a problem for accounting, but it would also be a problem if someone changed the amount to be paid to $1 instead of $99 for example and that you would send him the goods of his order even though the person only paid $1).
3. Not necessarily. You can for example use PayPal Express checkout and in that case, your customers can completely circumvent your checkout and directly checkout on PayPal. You can also remove the "login" and "address" views from your "checkout workflow" in order to not have any registration or address forms on your checkout).
But if you want the feature "guest checkout", which means that you want the customer to enter his address and email address, but not get a user account account created, then yes, that will require one of the commercial editions.