I'm not surprised they didn't roll over at all.
My suggestion:
Recall general orders regarding capture and escape:
"If captured, it is an officer's duty to escape. If escape is not possible, officers should conduct themselves such that the enemy must expend the maximum resources possible"
To that end, whatever can be done on our end for cheap that costs them more to respond to, the better.
The tone of the letter sounded pretty standard to me: "Behold my might, that none can stand against", which is yer basic tactic: make your opponent believe your position is unnassailable.
Whether it actually is or not is beyond me to determine, but requires a copyright lawyer.
As I understand fair use, it only applies to criticism or review of the work.
In this case, I think it's pretty clear from the context that Oleg's art is incorporated into an entirely new work, and is NOT a review or criticism of A Human Right .com, and the assertion that they are making the opposite point doesn't make it a critique.
I'm also thinking that because one of the images is Oleg himself, you'll have additional leverage.