@Ken Looking at the Aerial Acoustics 20T V2 (I'm guessing you have V1, so we can assume similar output), they are rated down to 27hz (+-2db point). That's basically full range. I can see how the JTR subs would be needed for your goals.
Your room is "medium" sized (2818ft^3), and since most of the JTR subs are ranked "extreme", you will have no problems in loudness/output in your room.
1. Yes, JTR Subs would be perfectly suited for your needs. My spreadsheet already shows how good they are, as you're already aware.
2. In terms of max output, the difference is about +-1db between the 2400 and 2400ULF...which I doubt you'll notice. I wouldn't base my decision just on this.
In terms of volume, 2400ULF is 40% bigger in size than 2400, which of course, you'll notice. Seeing how your room is not that big, I would assume a smaller 2400 would be more suited. But then again, you might want bigger subs...for bragging rights, of course. To each his own.
In terms of THD...here's 2400 (graphs from data-bass.com):
Here's 2400ULF:
I've only kept 105,110,115,120db, so you can compare both.
As you can see, distortion is worse below 20hz for 2400 model when compare to 2400ULF. Which is expected, given it's size. So if you want less distortion, you're going to have to get the 2400ULF.
3. Dual subs are always better than one. So that's an instant yes.
Again, in your room, I doubt you'll have any issues with output. So 4000ULF will likely be overkill for you in terms of output. So let's look at 4000ULF's distortion:
At 120db sweep, distortion is worse below 20hz when compared to 2400ULF's, but looks better elsewhere. So if you're not pushing this subwoofer to it's limits, it should have better distortion handling than 2400ULF...which of course, is expected given the 4000ULF size and price tag.
4. Nope, I think that's your endgame subs right there.
My personal opinion, given that you room size is not that large, my pick would be Dual Captivator 2400's.
But, if you're thinking of moving to a larger space in the near future (and assuming you can get away with dual massive subs), then of course I would upgrade to Dual Captivator 2400ULF's for an extra US$200.
Also
@richard12511 has JTR subs (Captivator RS2's) and he can offer some insights as well.