Hi,
About "frontend_old" file, it is still a file include in the package so Joomla will replace it during the update.
The core files should not be edited and it's quite better to use overrides instead but you're right in the fact that when you try to edit the "old" file, it does not create you automatically a "custom" one like it does for the "default".
We will fix it as soon as possible.
About the registration issue would it be possible to have more details about your configuration and about the created account before and after ?
Creating a guest user does not have any impact on Joomla and creating a Joomla user will link the user to the corresponding user with the same email in HikaShop. So it would be interesting to get more details about the "status" of the account with the issue.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.