It seems what ever design you contemplate employing a fee can always be gamed by an attacker so that he pays no cost.
The attacker can make his own DE and receive the fees. So then he attacks the other DEs (which are honest) 100% of the time but doesn't attack his own DE (or attacks he own less frequently) thus all users migrate to his DE. Thus he is losing much less in attack fees than he is gaining from fees.
Since more than one attacker can do this to each other, no one will use DE.
P.S. I knew that there would be a leak in fees that would maintain my conclusion that DE is fundamentally impossible. Sorry when I come to these sort of generative essence conclusions they stick.
By this logic no cryptocoin would exist.
Instead of all out war all the time, maybe some sort of truce would be made? Why would everyone running a DE want to spend all that time attacking the other DE's just to get attacked back and then nobody's works. Seems like MAD.
I notice your attack scenarios are getting more and more convoluted as now the attacker needs to run a DE themselves to make it even a potential issue. So things are improving. It also might be possible to make it so the attacker wont be able to get their fee back, but no script for that yet.
However, you must have missed my post where it is possible to identify the attacker. It would be after the fact, but that allows a reputation system to be created and as long as people are careful about filling orders from newbies, there wont be much problems.
James
P.S. Using your logic, no central exchange would exist as they would all be Ddosing each other all the time. You dont need to pay exchange fees to Ddos an website.