Speaker placement for 2-ch AND 5-ch same room

Wow! You increased the WAF by using 7 speakers.

Actually, the set-up you suggest sounds reasonable so long as there is nothing between the Odysseys (e.g., gear racks) and they are still away from the walls, and it sounds like they are. The soundstage may suffer a bit from having another set of speakers so close in, but you can check the effects of this by simply moving the Scenario's away (e.g., 5 feet or so) and see if there is any noticeable difference while listening to the Odysseys. If the speakers sound much better to you without the Scenario's nearby, consider having some easy way to move the Scenario's back and forth as needed for movies.

My first thought was that you should simply try HT on a stereo setup, but that big screen would force you to move the Odysseys only about 3 feet from the wall. That could also damage the soundstage and imaging. But, sometimes it doesn't seem to matter much and you might also want to experiement with that. If the Odysseys sound just fine in a position that allows you to utililize your screen without moving the speakers, you might want to sell the Scenarios and buy your wife a nice gift (hey, maybe there'll be something left over for that upgrade you've always wanted). You could still use the center and rears with the Odysseys, but you may also get away with just using a pure stereo mode. Experiment and trust your ears.

Blw, with the "remote" constraints, I can only say that you have done a masterful job managing the situation. You still may wish to experiment with moving the HT front speakers later (i.e., away from the main speakers when playing music and then back again for HT -- see if it matters and find a simple, wheeled?, way to do this if it does), but apart from that, it sounds like you are far from nuts.

Davehrab is correct. Dimensions which are exact mutliples do tend to reinforce or boost the same frequencies and create dips in some others -- in a 9x15x21 room, those boosted frequencies are at 188 Hz and 377 Hz with some fairly big gaps or dips in the ranges of 81-107, 161-188, 188-215 and 269-296. If the dimensions are off a bit (e.g., 8'9" rather than 9'), it will affect these in a relatively positive way. My understanding is that anything above about 300 Hz is not a big deal so the only real peak is at 188 Hz and that could be managed with specialized treatments. The 27 Hz gaps might be a bit of a stickier problem. Rives knows more about this and will undoubtedly have more to add. Feel free to ask questions though and I'll give it a shot as will other more learned Audiogoners. Good catch Dave.

Try the book Sound Studio Construction on a Budget by F. Alton Everest. Good stuff. BTW, don't drop your ceiling yet, I was talking in generalities. I'll be happy to send you a spreadsheet on these calculations so you can test different dimensional impacts. Let me know if you are spreadsheet savvy and I'll work on it over the weekend to make it a bit less subjective and easier to use. Cheers.