Look, if this is the space, then this is the space. I wish people would stop giving you crap about what you are working with, and just be helpful in providing suggestions, options, and solutions.
I certainly feel that if you have a soffit near the screen, that putting in-wall speakers, or on-wall speakers facing the seating position is better than in-ceiling speakers facing (mostly) downward. That said, I've used the Monoprice in-ceiling speakers with angled drivers, and they do a good job of directing audio more towards the listening position than normal in-ceiling speakers with drivers facing straight down.
I like the idea of you using in-wall speakers for the surrounds, and look at one of the several speaker position guides on the main Audioholics website to get a good idea for best location.
I'm not sure I would add any ceiling speakers for Atmos, but it isn't the end of the world if you do or don't. That's up to you.
I would say, that I wouldn't go overboard on getting 'great', or much better than 'good' in-wall/ceiling speakers. You have to drop $500+ typically to get quality in-ceiling/wall speakers that have a real bump up from sub $80 in-wall speakers. Using $50 8" or 6" 2-way (or 3-way) speakers isn't the wrong way to go here.
Your subwoofer selection should be completely separate from your speaker selection and should be very carefully considered. Wireless options for subwoofers are out there and can be added to almost any subwoofer on the market. So, get a GOOD subwoofer, regardless of the speaker choice. It will help fill in the low end and add a solid presence to the room. This can't be said enough: Subwoofer selection is completely separate from speaker selection, and most reputable speaker companies are not reputable subwoofer companies, and often, vice-versa.
Since nobody else asked, what is the recommendation for a screen and a projector? Since those can be big-ticket items that cost as much, or more, than the speakers, it would be good to know your plan there. Specifics are good. IME, a tab-tensioned motorized screen is the only way to go. If you aren't going to spend the money on a good tab-tensioned screen, then get a inexpensive manual screen. It will save you a lot of money and provide the same results as a more expensive motorized screen.
How far are you sitting from this screen? (eyes to screen?)
Typically you want about 10" of diagonal for each foot of viewing distance.
What make/model of projector are you considering?
In a VERY poor white room with white ceilings and a light floor, using a really high contrast projector is pretty pointless. Instead, consider one that is mid-contrast and has the proper placement flexibility for you. The Epson 3200 (USA) would be my recommendation in that room. Decent contrast, 4K support (faux-K), a good lens, and great placement flexibility.
