I'm not sure what you want us to consider regarding responsive design ?
HikaShop is already responsive. We cannot ask PayPal to change their PayPal Standard form to be responsive (we can but they'll just ignore us).
If you want a responsive payment gateway with HikaShop, that's already possible. Just use one which supports that.
Now regarding PayPal Express Checkout. The goal of the Express Checkout gateway of PayPal is to skip the checkout on your website so that you only see the PayPal checkout page and HikaShop then takes the information that you entered in the PayPal Express Checkout and adds it to your order on the website after the payment.
With other payment gateways, the process is different: The checkout is handled by HikaShop and it's HikaShop which sends the information of the order to the payment gateway.
So normally, when you use the Express Checkout, you want to but the button right at the beginning of the checkout or even in the cart module so that your customers can skip the normal checkout of HikaShop and directly go through PayPal's Express Checkout.
If you put the button at the end of the HikaShop checkout, it defeats the purpose of the payment gateway.
If you want a responsive payment method with PayPal, I would recommend to look at PayPal Advanced (yes, they have a dozen of different gateways, each time with a different API...). We just finished developing a payment plugin for it and it will be included in next version of HikaShop, in May.