Hi,
1. Turn off the corresponding option in the options page of the Joomla user manager.
2. You can create a menu item of the type HikaShop user download page via the Joomla menu manager and turn on the "Enable customer download area" setting in the HikaShop configuration.
Then, get the link of that menu item on your frontend and set it in the "return URL" parameter of your payment method and your customers will be redirected there automatically after the payment.
3. It would require the development of a custom plugin. So it's possible to do it, but I don't know of such plugin already available for HikaShop.