Hello,
1. Can you give us more information about that login error message through some screenshots for example ? Thank you.
Also, note that Hikashop use Joomla users, so the login process is related to your Joomla.
2. To display coupons on your checkout page, the solution will be to directly add a "coupon" step through your checkout workflow via "hikashop->system->configuration->checkout".
3. The shipping step always display the first shipping method available, the solution will be to add the shipping action through a different checkout step than the login action on your checkout workflow.
Kind regards,
Mohamed.