No, every question of the survey is optional and can freely be skipped.
Yes, all data will be released publicly, but only after being purged of any identifying information (emails, usernames, IDs, etc.).
Surveys are designed based on community feedback, as well as reviewed by experts. You can learn more here.
Results are generally released a few weeks after the survey closes.
Surveys are run by Devographics together with a collective of contributors, translators, and volunteers.