I think the height adjustment is going to come down to personal taste.  So if +2 works for you, I would stick with it.

I don’t have personal experience with those Sonance speakers, but generally speaking, you don’t need a lot from your rear surround speaker, so I would opt for the 6”. The location of the speaker, at seating height behind you, is more important.  Sounds like you’re forced to use ceiling speakers for aesthetic reasons?  

I don’t know what you mean by a “300 type atmos ceiling speaker”.

Era 300s. 

Yes, I figured that part out, but I don’t know what an Era 300 on the ceiling would “look” like.  Do you want it to be a powered speaker mounted to the ceiling?  Or passive speaker connect to an amp?  Is it supposed to have multiple woofers.tweeters to provide different channels of audio bouncing off the walls as well as a height channel?  If it’s passive, that require multiple speaker wire connections (for each channel) or a new specialized cable...and a new amp.

No the era 300s would have to be on stands behind the couch, which the wife does not want, hence the ceiling Sonance. I was saying it would be cool if Sonance or Sonos had era 300 dolby atmos capabilities in a rear ceiling speaker. 

Any chance the Sonos in-wall speakers would work at the rear of your room?