Netflix etc. online streaming is stereo through a web browser even if you configure your PC for surround sound modes. The best you can do through a PC is use stereo mode in the PC setup menu so you don't get some weird upmix generated by the PC, and use the upmix in your receiver instead.
You need to use a Roku or smart TV if you want to play online streaming in surround format through your receiver.
The manuals for your HTIB are at this link: https://www.onkyousa.com/Products/mo...&class=Systems
You want to download the basic/advanced manual to find the Atmos configuration instructions: http://filedepot.onkyousa.com/Files/...171219_web.pdf
After you open the PDF, click on the 'Advanced' link on the first page and it will put you at the table of contents for the advanced manual portion of the file.
Your receiver allows you to decode top rear speakers. You can make your rear speakers do double duty if you put them up high and aim them downward toward your ears. Then if you have sources with Atmos, you can decode the top rear channels, and if you have sources with rear speakers, you can decode those too, and both types of content will be reproduced somewhat accurately.
Alternatively, you can also put them above either your front or surround speakers to decode them as other ceiling channels, but you will lose compatibility with legacy 7.1 bitstreams and either the rear information will end up in the wrong place or you will need to enable a synthesis mode instead to create Atmos-like height channels from decorrelated content (ambient reverb, wind/rain, etc.).
If you put the ceiling speakers on tall stands or tall bookshelves, or on shelves installed at ceiling height, you will likely get superior performance compared to 'Atmos' (a misnomer IMO) highly directional speakers that aim at the ceiling and bounce the sound back down at you off the ceiling. Low frequencies are not directional from such small and simple speakers and the height imaging will suffer with reflecting designs. They are convenient though because they are simple to install. They sit right on top of your front or surround or rear speakers and aim up at the ceiling.
For HTIB in your setup I would not spend additional money on reflecting speakers since your rear speakers are small enough to install near the ceiling anyway. I would definitely put the rear speakers up high in the location you indicated to preserve compatibility with 7.1 bistreams. I would only suggest that you move them closer toward each other because bass-limited channel speakers tend to sound boxed in from the sidewall reflections when placed in corners. The reflections muddy the imaging with additional 'sources' mirrored off the walls, and the delayed sound from those reflections causes phase cancellations between direct and reflected sound that manifest as combing and lobing and colors the frequency response.
You have the same reflection issue with speakers in a bicorner near one wall and the ceiling anyway, but having them in a tricorner near a sidewall too just makes the problem worse.
You are likely going to have much more legacy 7.1 content available than Atmos unless either you subscribe to a streaming service with Atmos, or you invest a substantial amount of money in new media. It would probably pay off to maintain legacy compatibility, but maybe not if you are just getting started.
Also, the immersion of a pair of rear speakers from 7.1 content or even synthesized from 5.1 or 2.0 is superior to the immersion of a single pair of synthesized height speakers that are close to the front or surround speakers IMO. Rear speakers mounted high up are going to do more for your total experience than front high speakers or top middle speakers with most legacy content.
New content with Atmos from streaming services is likely going to have height information that decodes best on your receiver through a single pair of top middle or top front speakers. Top middle puts the height information up where it has good separation but requires special care for mounting.
Top rear or rear height decoded speakers behind you on shelving is probably going to be your easiest solution given the room layout you provided. I would probably default to that just for the sake of practicality. That's usually what dictates speaker location IMO. They end up where they fit regardless of what that does to the sound.