Opposite the listening position is the norm. You normally have music playing towards you from in front: if you were watching a live performance it would be odd if the sound came from behind you. 

The mantle would be the easy option but as said the stereo image wouldn't work well.

Beside the wide doorway, on stands would be far better for sound but a bit more intrusive. Same place but wall mounts might be an option.

How do you listen? Some people sit in a favorite spot and listen to music and that at is the activity. These people benefit from a focused setup. Start the placement plan with an equilateral triangle listener-left-right arrangement. Other listeners are always up and about. In this case placing near the sofa would cover both rooms better than the mantle or beside the door.

If you try several positions that aren't good for stereo imaging you might also consider not Pairing for stereo, instead Grouping for dual mono.

Honestly we usually use that room for visiting with guests, family games, kids dance parties. For the most part, background music.  We're not audiophiles who will notice little things.  Mainly I'd like to set it up so that there isn't a speaker next to someone's head on the couch, etc.

Didn't even think of that! Thank!

Yes I agree.  Having a hard time figuring out how to keep the setup out of the way but still opposite the couch.  That's kind of where my head was at with mounting them on the wall, I just wanted to check and see if that was a terrible idea prior to doing the electrical to add the outlets up high for them ..

If use wall mounts, it would be above where the doorway is..  I would love to use the mantle but thought I should spread them out more...?

Do what sounds best to you. The nice thing about Sonos is you have that option. 

For background on the mantle in Mono Mode would be good.

Keep in mind the triangle arrangement needed for getting a decent stereo image.

Also on the mantle with some wire covers you’d never notice them.

Very simplified:

Why the insistence on placing the speakers high up? Would wall mounting at ear high on both side of the passage to the sun room not fit?

Just visually my wife doesn't like the idea, that's all.  Trying to get decent sound and have a happy wife :)

High isn’t great for sound, if not the mantle, maybe on stands and Mono mode at the ends of the couch?

Might be a bit loud close to the speakers but the rest of the room should be reasonable.