Adding a Drywall Partition

I wouldn't think of it in terms of absorption. There will be very little "absorption" in drywall walls and all of it would be a frequencies so low as to be of no real concern.

Since you mention it's temporary, I would not do this as it doesn't add anything but symmetrical reflections. I would deaden the primary reflection points as much as possible and move on. You also didn't mention the distance between the ears and the side walls. If they're more than 2x-3x the distance of the ear to the monitors, then I wouldn't worry. I have no proof to back that up, just that I've worked in a few small studios over the years and never really had an issue as long as the walls are either dead, or far enough away that they don't cause immediate reflections.

But to answer your questions, I would insulate the wall simply to keep resonances down, but since drywall is quite damped on it's own due to it's weight, (if your studding is normal 16") then you're probably not going to have much issue anyway if you use 5/8 drywall.

Also, drywall walls are not meant to "absorb" sound. They're meant to keep sound from transferring out. This means mass. Heavier weight means lower frequencies being kept from transferring through the material by lowering the resonance point below a reasonable frequency. The less mass a wall has, the easier it is to transfer the energy into the material and cause it to resonate.

On the other hand, this is important for something like a live room where the power of the sound is 120dB, but not really that important in a room where the audio power is only 70dB.

Plus 1 to that.  By making the room smaller you may be making it harder to treat.  On paper you may end up with better dimensions, and a better ratio of HxWxD but instead of loosing a "hallway size" space on the other side of the partition it would most likely be better to fill that space with a HUGE floor to ceiling bass trap/wall.  Of course I have no idea the layout of the room, but I'd reconsider this plan if it were me.