Dear Anshita,
have you any details on error messages the user gets when trying to sibmit using the HTML file attached to the mail?
For the BSP scenario please talk first to your security officer if you're allowed to expose this service to the Internet. Then maintained an Alias in transaciton SICF for the Survey BSP application providing a technical user with the needed authorizations. Then everyone receiving the link should be able to fill out the survey. I'm using this scerario for over 10 years now.
Best regards
Gregor