Hi,
1. That's fine. This is exactly what you want: the user is removed in Joomla (so the user doesn't have a user account) but is still listed in HikaShop. Next time the user comes back, he will be able to guest checkout with his email address.
2. By default the "Auto select default shipping and payment methods" setting of the HikaShop configuration is turned on and thus this is not a problem. If you turned it off, then yes, customers would have to select their payment method and thus you can't hide it.
Turn back on that option.