Okay starting from the top.  There is no real issue with the back wall opening, or the missing wall to the right of your listening position.  The tricky part for you would be sub placement as you cannot corner load or ¼ wall load. And rear placement might be alittle close to your listening position which could give you a directional sound. But hard to know until you do a “sub crawl” (look it up) and try different positions yourself, plus Truplay with an IOS device. 

I own the full Arc set up, and a ps4 pro.  Within the Ps4 settings I have my settings like this. Sound and Screen > HDMI OUT > Linear PCM. In the HDMI option select output in 5.1ch option/box. Do not leave it on the 7.1 default.  Games always play in surround for me with no missing dialog or effects.  You will lose effects if you do not select 5.1 manually. 

Unfortunately I cannot help you with the DD+ issue with the Samsung, but Sonos doesn’t support DD+ anyways, just DD (IIRC).  This has never been a problem for me when streaming, or gaming from a PC, or console of any kind. 

You cannot have the Arc with additional front speakers and surrounds. The Arc is a LCR with Atmos. 

I am unfamiliar with the Arcana since my Tv has earc, cannot help you there either. 

Use Truplay, and when you do stay mostly where the couch is, do not get in front of the coffee table / near the tv and do not go into the other room.  This will give you the most centered “bubble” of Atmos sound.  I have had great experiences doing Truplay this way, rather than the recommended “walk around the entire room” method.  Which gave me awful results, my comparison. 

Thanks, this is really helpful.

Please if you would a couple of more questions.

Regarding this:

I own the full Arc set up

and

You cannot have the Arc with additional front speakers and surrounds

What do you mean by full arc setup, is it arc + 5 speakers and subwoofer or something else?
It would seem that it is something else going from second quote.
I thought that arc could be hooked with multiple speakers and subwoofer ? If not, is there another Sonos product that i could use to get 5.1 setup ?

Or do you mean that i should connect arc with 2 back speakers and subwoofer without front speakers ?

Rear placement + subwoofer

Considering the issues you mentioned with subwoofer placement and rear placement speakers, do you think that it would be best to skip any of those ? Could it still be worth it assuming i make proper use of Trueplay tuning o_O ?

Also, what does LCR stand for, google is not really helpful :)

I guess i am also worried about lag with the PS4 pro and 5.1 setup, but this could be related to the Samsung TVs.

Thanks again

The arc itself is your left, center, right set up. You cannot add two more channel up front and have the sound bar be a center only. But yes, you add two additional speakers as rear surrounds only. So you have Arc, two speakers(surrounds), sub (or two). Just like you mentioned at the end of your first paragraph. you can easily wall mount the rear speakers which I highly suggest and have them facing towards you as much as possible. I use the official Sonos mount as they have a pretty good range and it is fine if they are slightly off axis, since it's a diffused sound anyways, for movies. I do not have any lag with my PS4 pro in gaming, but I do not use my console for streaming. 

Ah great, that is really helpful :)