I took a look at that discussion. The attachment between the OEM hitch and the Ascent unibody frame does not look like it can transfer the moment loads from a wdh to the frame. And obviously Subaru does not authorize it. So in that I agree with the author, you shouldn't use one.
However, his next statement, that the Ascent has design features so that it wouldn't benefit from a wdh, simply can't be true. Basic physics is basic physics, the tow vehicle is like a teeter totter pivoting around the rear axle, so without a wdh the load on the front axle will be reduced when a load is applied to the hitch. That is inherently not good, and there is no magic frame and suspension design that can fix that. A wdh does.
As for antisway, adding sway control might confuse the vehicle's electronic sway control, which is going to be using accelerometers and/or load sensors to adjust braking action at the tow vehicle wheels. However, only friction based antisway or systems that control the brakes on the trailer are going to directly function to keep the trailer behind the tow vehicle once sway starts. So, I'd look into that further before making a decision. Both friction and brake based antisway systems are available separately from wdh.
We did set my wife up with cushions and that helps, but its not like having a proper office chair. I'm sure you could get a chair through the door by partially disassembling it if it didn't fit. But where to put it? Someone on the forum might have a solution, you could try asking that question specifically on another thread.