Reality is not as strict as theory. I've discovered this time and again with less than ideal speaker locations. Don't worry a lot about positioning that's not perfect. The best you can do will probably be okay.
On the other hand, you could get some surface mount speakers and position them in the room up on poles that hold them where you'd mount them. This would allow you to move them and find (I'll predict) that major positional differences make smaller sonic differences than you'd imagine.
And anything you do will help you get a feel for the effects in the actual room.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
