Hi Matt, welcome to the forum!
First off, let me ask a question. Would in-ceilings be an option? If so, that would probably solve all placement issues - if you want to go 7.1, I'd put a pair just behind the sectional and the second set back by where you have the question mark placements (probably about two feet or so in from the wall).
If in-ceilings don't work, then back to our original programming on the 5DB/5B option. In which case, I have a question or two for you.
First, the LS and RS positioning - can that be moved back (toward the back of the sectional) a bit? If you can place them directly to the sides of your primary viewing position (which I am assuming to be the sectional where you have the word "sectional"). In that position, the 5DB would be awesome since they'll direct sound both ahead of and behind you creating a really immersive sound field and envelope you pretty nicely while watching movies.
For the rear surrounds, however, I would probably stick with standard bookshelves - the 5B's since if a 5DB is placed behind you, you lose that really good immersive effect as all the sound is coming from behind you. In which case, a standard bookshelf will give you mostly the same effect for less money.
As for the distance to the rears, I think you should be ok at 15' with the 5B's. It still is within the soundstage of them and you should still be getting good effects sound from them while watching movies. And they'll certainly work very well for music when sitting at the poker table.
Hope this helps!
Mark
Aperion Audio Guru