I have no experience with this guy, but it claims to do what is needed.

This would be a great solution for other people that miss the possibility to have headphones connected while having Sonos HDMi soundbars connected.

Hmm. I think an actual switch is required for the purpose, otherwise the signal will continue to pass to the Arc. I’ll keep looking, thanks for the tip.

Generally, a switch chooses from several inputs and directs to one output. You have a single source that you need to direct to several outputs.  I think that's more like a splitter. 

Could you not mute the Arc?

Possibly, but it seems likely that the CEC would mute the TV. If the Arc can be muted without muting the TV, a splitter that passes through eARC would work.

The best solution found:

A smart plug for the Arc, BT headphones paired with the TV. Withe Arc powered down, no problem.

It’s unfortunate that the CEC specifications for ARC and eARC don’t allow for headphones, just a single sink (speaker). 

I expect that you will be able to Mute ARC or at least lower its Volume to zero. Powering down is liable to upset the CEC chain, resulting in the TV shutting down.