I believe your understanding is incorrect. GDPR certainly includes storage and processing, both of which backups probably trigger.
Anyway, think about the spirit of the law, and then think about how that interacts with backups. If someone asks to be deleted from your system, you do so, and then you restore a backup with their data, you have clearly violated the intent.
Keep a log of deleted users and re-delete upon restore.
The GDPR contains exceptions for data storage for which it is infeasible or outside reasonable effort to delete individual records or you have legal compliances to uphold.
No since the GDPR exempts things you need for legal compliance, thus a list of users who have asked to be deleted is fine if it's being used to ensure compliance.
