passwords: completely secure screen lock in Ubuntu with encryption

As I understand it, what you are looking for is a combination of a few different things that are probably too broad for a single question.

This should give you an idea of ​​how to encrypt on hold in Linux.

Is there a secure screen lock (no errors, no vulnerability against X11 locks, etc.)?

Nothing is totally safe and immune to errors or vulnerabilities. I would like to compare prices and weigh different options. One thing to keep in mind is that you may face incompatibility issues between a custom lock screen application and your desktop environment (if applicable).

Can I have a boot encryption in addition to the full disk encryption and configure the screen lock so that the same password unlocks and decrypts the boot drive (I don't want to enter two passwords)?

This will depend on the implementation of your particular lock screen and the encryption settings.

Protection against cold start attacks and access to the physical port or manipulation are other problems with a large amount of written material at your fingertips. You will gain more traction by doing a little research on each one, selecting specific areas on which you want clarification and asking them as separate questions.