Are there passive attenuators that don't zero volume and only offer one input ?

Which is to say it is essential I should think to be able to zero volume, and have more than one input, do you agree ?

No.
Every contact, additional component and wire/trace affects the sound. Anything below -20dB is pointless with properly configured gain structure.

Is it HiFi or sonic wallpaper?

Just had a look @ OP Stereo Coffee LDR attenuator @ 
Stereo Coffee Passive LDR Preamplifier (wixsite.com)

Why an A/C supply when a DC wallwart would keep all hum outtadabox?

PCB looks like something a kid would have designed 40 years ago.

TRIM POTS?!?!?!? WTF? Trim pots are the bane of high fidelity.

"However, the real magic of the 2020 StereoCoffee is what happens after 40 to 50 minutes playing music."

If a passive is changing, it's not passive. The whole point, at least for me for 40+ years, is PASSIVES DO NOT CHANGE!!!

Rectification can then be achieved with thyristors, and jfets

¿Que?

Thyristor is a gated diode. What earthly advantage is there over a 1N400x diode?

High quality DC supplies are 10 a penny today.

±0.1VDC 1mVDC ripple is hardly anything to sneeze at.
Thyristors make gawdawful noise when switching. Pointless to control current in a device using a few mA.
Quit digging.