Lots of things going on here, but creating an enjoyable and balanced mixing setup shouldn't be difficult. Here are a few things to remember:
1. Room modes always exist and we don't want the least amount of room modes, we want the most even distribution of the modes. In fact, an ideal room would have tons of room modes, like 3 modes for every 1/3 octave, even in the low frequencies. That's what the Bonello and Bolt indicators show in the Room Mode Calculator - the beneficial distribution of modes. REW has amazing tools to predict your room's response and then measure it. The Room Mode Calculators are a great help in learning which boundaries contribute to each mode.
2. The biggest problem I find in commercial and project rooms is not poor frequency response, but poor decay time. The decay time must be similar at all frequencies or the room will sound confusing. Don't treat your room with only 1" or 2" (or even just 4") acoustic panels or your bass decay time will be so much longer than your mid- and high-frequency decay time and your room will have no definition. You are hoping for a room that has a balanced frequency response from 30-ish Hz up with a variation of plus or minus 3 to 6 dB. Don't expect flat - that just rarely happens. A room with an even decay time at all frequencies will sound amazing!
3. The keys to starting our right are: 1. find the best listening position, 2. Find the best speaker location 3. For smallish rooms, you'll need about 10 square feet of acoustic treatment (depth to be explained later) for every square meter of floor space. (sorry for mixing units, but that's how my brain is thinking right now)
From your drawings, I like version D, except for the symmetry (extra room on the door side. I would build a solid wall (maybe with more storage) to continue the wall to meet the edge of the door frame. (see image) Then cheat the desk toward the door to create more symmetry. Option C could work if you built a front wall diagonally across the corner to match the angle of the desk, but that layout would be hard to predict and might be awkward for having space to walk around the room.
Now you have symmetry and some good broadband absorption in the front and back of the room.
If possible, do not put drywall on the ceiling (unless you need it for sound isolation purposes). Take advantage of that space to put insulation in the ceiling. If you have more than 8" or depth, you can use inexpensive roll insulation and cover it with thin plastic and then fabric. If it is 4" - 8" deep, I would use Roxul Safe n Sound and also plastic sheeting and fabric. If less than 4" use a denser insulation board, like OC 703 or 705.
If you can do this ceiling treatment over the front 40% of your room, the low frequencies will be very well controlled and the room will sound well-balanced. If you need to cap the ceiling, then build a large cloud of 8" Roxul SnS to cover as much of the front of the room as possible.
Treat any first reflection point on the side walls with 2" to 4" panels. You will probably wind up with 4 to 8 (2' x 4') panels scattered around the room to stop flutter echoes and first reflections.
Find your ideal listening position! You can use the REW Room simulator to move the listening position and speakers around. Your speakers should be as close to the front wall as possible, with some 4" absorption in the area behind the speakers to avoid SBIR problems. You can also find the ideal listening position by putting a single speaker in the front corner and playing some music and start around 38% from the front wall and slowly roll your chair forward and back (as much as a few feet in either direction) to find the most balanced listening position. I usually start with the REW prediction and then do a listening test to fine-tune the location.
After you dial all this in, then shoot the room with REW and look at the waterfall graph to find any areas of long decay time. Your room should be between 150ms and 250ms from 50 Hz up. If you have a hot spot in the bass, you may then want to build or buy some tuned traps to address a specific low-frequency mode. Remember that the tuned trap must be placed at an area of high pressure, ie, against a wall that contributes to that modal resonance. I would expect to need about 20 sq feet of tuned trapping (the area of the trap facing into the room) to hear any measurable result.
So, basically, 1. build a symmetrical layout, 2. find your listening position, 3. locate your speaker (probably close the front wall) creating close to an equilateral triangle with spot 1 foot behind the listeners head and center of each speaker's face, 4. Treat the room with broadband absorption down to as low as you can manage (8" to 12" insulation depth), 5. After all this measure the response of your room and treat the frequencies with the longest decay time.
Hopefully, this was concise enough, but also complete enough!