How is digital not a ‘complete signal’? Digital recording creates a blueprint of the music/electrical signal input and then rebuilds it on playback. If done correctly it is capable of more accurate capture of music.

1. You are right about specification. There are many variables. And bounding between subjective impression and objective figures is complex enough.
However, figures need for development to detect right improvement direction.

Also for more exact equipment estimation, need more figures than published in available specification.

Example: THD as itself say almost nothing about sound quality. Here need more information about spectral content of the distortions for all input frequencies.

2. Analog sound is more distorted than digital one.
Theoretically, we can create “analog” sound from digital stuff. But we can’t create purer digital signal from analog source.

You are right about specification. Bounding between figures and subjective impression is not so simple. Because there are many variables. There are not any mystique. All these things may be objectively researched and measured.
And we (developers) can use fugures to detect right improving direction.

Analog mediums are nightmare for developers. Because there are so significant distortions comparing digital ones. However, subjectively distorted signals may sound better, than purer signals. But it is not matter of better sound quality. It is matter of art.

Theoretically, we can model analog signal (its distortions) from digital one and get same sound. But we can’t create purer digital signal from analog one.

DSD is not type of DRM (Digital Right Management).
DRM is kind encrypting of digital audio content to copying protection.
DSD is kind of digital signal modulation. It is not encrypted information. There is not copy/demodulation protection. DSD is one of implementations of sigma delta modulation. And DSD is converted to PCM or analog via usual sigma-delta demodulator.