Correct: the Sonos speaker system “replaces” the tv speakers.: if plugging in the eArc doesn’t do that you’ll need to disable them in the tv menu.
 

If you want full Atmos you’ll need to bond the Era 300’s as surrounds and also have the Sub bonded to the Arc. That, with the exception of a second Sub, is as much as you can have in that one “room” - which I think you’ve called Living Room. The Arc provided the 3 front channels. 

If you want a wider soundstage for music listening, you could set up your One SL’s as a stereo pair called Lounge. You can then group Lounge and Living Room to have the same sound playing through all speakers. Note, though, that there will be a 70msec delay if you use the TV as a source and play that to the group. This is down to how Sonos achieves sync’d multi-room music play. Music, streamed or from your music library,  will be in sync. 

Hey Nik, very helpful my friend. A couple more clarifications. So, for the One SL’s, do you suggest putting those in my book shelves on the right and left of the Arc, assuming so. I like your idea of setting them up on their “own “. If I do that and have the Arc, Sub and Era 300’s configured as Surround, when I am streaming music from my library and group the One’s, does the entire system go in to Stereo mode?

Does that make sense? Did I asked that correctly… 

Said differently, when I am done watching a Atmos movie and then want to now listen to Spotify, Amazon etc. music, what do I have to do? Sounds like just grouping the virtual “lounge” and the living room

To play music, just select it as the source in the app on your phone.
The Arc, Sub and rear surrounds will play music too, so you could have just them in Living Room, and use the OneSL’s elsewhere. You’ll get music through the surround speakers too.  Most soundbars lack “width”,  and that was why I was suggesting them for music. But the system is easily reconfigured so you can play with different setups and decide what you like.