Agreed, it’s usually fstab. I wrote a blog post once to remind myself how to fix it, which may be helpful here: Procedure to Correct /etc/fstab File for Resolving Ubuntu Emergency Mode.

Yes, they allow registration while connected to VPNs.

These are both decent guides:

• https://www.privacyguides.org/en/os/ios-overview/
• https://github.com/iPrivacyGuides/iOS-Privacy-Guide

For your ad blocking question, AdGuard is my recommendation.