" I was planning on sitting 38% from the rear wall as instructed by Ethan Winer of Realtraps fame"
Someone telling you they think 38% as a ratio for you to chose I think is rather blanketed and "uninformed" personally! It all comes down to measured response, and uniformity across the seating areas (or at least main seating area). You may very well put a row, or so, of seats at 38% from the back wall, and such--but what does the overall measurement of the response curve measure from there?! Reasons why a blanketed number like that will never work (unless you get lucky as heck), is that all rooms are different, and all speakers are different! This means the actual combinations of variables will ultimately determin.
You really need to measure and listen from different locations to pinpoint the best seating locations. All the guessing and "blanketed formulas" and ratios in the world won't mean anything if they don't actually yeild good sonic foundations!! Basically, you gotta go through the steps. Otherwise, TAKE A WILD GUESS, stick some seats where YOU THINK they might go (that goes for the speaker possitions as well, AND HOPE FOR THE BEST!!! Well guess what? It most never works out as well as you hoped! Like you said, there's a lot of issues to adress, and variables to consider.
Basically, sounds like you'll have one center seat for you to do "stereo" music listening, so that will be a "compromise seat", so you can get your "imaging". But be warned, you'll have the issues I mentioned there.
Personally, if I were you, and I had some budget to play around with, I'd STRONGLY CONSIDER contracting someone like Rives Audio to at least plan out your seating, speaker placments, and acoustic treatments for you room, and take a measurement or two-at THE VERY VERY LEAST! This couple thousand investment will yeild you better results than 10's of thousands spent on even the best gear money could buy! This is no understatement. Otherwise, some other competent acoustical engineering company. It's a thought.
Basically, the truth is, you'll get down to GUESSING where everything goes, and not knowing anything about what's doing what in your system. Otherwise, consider lots and lots of reading, and trying things for your self.
good luck