Hi,
1. Regarding scenario 1, I think there an issue with how the product for the renewal is configured and because of that, the add to cart is not successful. I'm not sure. We would need a backend and a FTP access to look into it.
Could you provide that along with a link to this thread via our contact form ?
www.hikashop.com/support/contact-us.html
2. Regarding scenario 2, this will happen as HikaSerial doesn't change the current menu item when directing to the checkout.
In that case, the "force a menu during checkout" setting in the HikaShop configuration is supposed to self correct the current menu item when the checkout is being accessed from another menu item.
However, it might not work properly based on how the checkout SEF name setting is configured. Indeed, if the checkout SEF name is the same as the menu item, then when the URL is accessed, the system doesn't know anymore if it's the SEF name or the menu item name. How did you configure the checkout SEF name ? If you change it, does it help ?