Hi

In any space if mounting a rear speaker too high you’ll lose the sonic response you hope to obtain. That’s why stands are preferred in some situations.

My suggestion is to go to the FAQ for each speaker you are considering and review the product manual for wall mounting guides for distance.

Regarding the Era 300 or Era 100 the latter will not produce rear Dolby Atmos effects.

Also note that the Arc, Beam2 and Era 300 depend upon side wall and ceiling reflections to produce the simulation of Dolby Atmos effects. Unfortunately neither of those speakers perform well in a space that is an open “park” landscape with Dolby Atmos. Normal surround effects (Dolby Digital 3.0, 3.1, 5.0 and 5.1) are more forgiving.

In any situation Trueplay (using and iOS device) may help with adjusting room sonics. However it also may not perform well in too large of a space 

Note: To get the best performance from your Arc (IMO) add a Sonos Sub (full size for your space). The Arc will be relieved of  low-end duties and produce better mids for music and vocals; especially so for movie dialogue. I’d take a sub over adding surrounds; if I could only select one (given my experience with Sonos soundbars).

My Play 1 rear speakers are about 7 feet up, far too high and too close together, but the system sounds better with them than without. I tried them lower (in a non spouse approved position) and they did sound better at the proper height.

I’d do it right if at all possible but even wrong sounds better than none to my ears.