Placement question please

The 300s are likely to sound best even if the placememt isn't optimal, I'd go with them on some good quality stands.

Era-100s would work there but I think you would notice the difference.

I had 300 placement issues here but even in "as good as I could do" spots they sounded good. Spouse was still not happy with the 300s in "her" room so I swapped in 100s. Not unhappy with the 100s but for Atmos content the difference is noticeable. 

If I place them behind the couch they are right by our heads.  Are they too loud then?

You can adjust the volume levels as needed so no. Also if you look at the driver layout, the Sonos store has a nice one, you can see the sound is directed to the sides and up.

In addition the front driver is disabled for surround use to reduce the chance of overpowering the front channels.

From the forum search bot:

Summary

When an Era 300 is configured as a surround speaker, its front-facing (center) driver is disabled by design to keep surrounds ambient rather than producing direct, localized sound (era 300 central speaker off when connected to beam).

Because of that design choice, dialog and center-channel material remain routed to the soundbar (not the Era 300 surrounds) in a home theater setup (era 300 central speaker off when connected to beam).

Our temporary home poses some unique challenges as it’s wide open making TV viewing surrounds difficult to place. I’m about to purchase an Arc Ultra and thinking of adding 300s but given location of seating (red arrows) I’m unsure of placements? 

 

As it’s only a temporary home, why not go without the surrounds? 

Our temporary home poses some unique challenges as it’s wide open making TV viewing surrounds difficult to place. I’m about to purchase an Arc Ultra and thinking of adding 300s but given location of seating (red arrows) I’m unsure of placements? 

 

As it’s only a temporary home, why not go without the surrounds? 

because I have a single use discount that has to be used by end of year graciously provided by Sonos due to the fire.

You can never* have too many Sonos! In my big open room I have the TV set - Arc Ultra, Sub and 100s, a 100 Pair, a 300 Pair, a Play and Play 5. My "spousally declared excess" 300 surrounds became an Atmos/stereo pair in another room and gained a Sub, the old Beam is in a bathroom. Even my antique Play 3 Pair and Sub have a home in the garage. 

I must admit sad puppy eyes from the grandkids did get my original Arc and Play 1s re-homed to their basement TV room instead of my spare bedroom. 

*It does get a bit complicated past 32.

For your discount, consider a set like the Ultimate Immersive Set with Arc Ultra. I Do wish they offered a dual Sub set too.

https://www.sonos.com/en-us/shop/ultimate-immersive-set-arc-ultra-sub-era-300-pair-black

A single Sub will sound great there, a second would tempt me in that space.

A Pair of 300s or 100s in the kitchen would be nice too.

Ahh, so get the Era 300’s. After testing they work ok, box them up until you get to your regular home. Or use them in the kitchen, as already suggested. They are fantastic as a stereo pair.