I Fully understand your worries about the cheaters however the whole theory itself fails for 2 reasons as i have stated before which are:
1. The cheats which bypass ACE does bypass all the other anticheats afaik, as for nexgen it's not even an anticheat and that's what totally makes the theory wrong. nexgen would only kick a player in the 2 cases i mentioned above.
2. It's true that there is a cheat which bypass ACE however screenshots from "the suspected player side" and demos of course help
It's true that it's hard to recognize triggerbot from demo, however if you are experienced enough you can know.
I'm a bit confused right now, i thought you were asking for a help about this issue in your server? but now you have mentioned our server so not sure if you are talking about the "cheaters" in your server or the "cheaters" on our server. anyway let me clear it about the demo... it's true that spectator is locked in our server so you can't watch someone with behindview however you can still do a demo and watch the player who you want to watch in behindview in this demo.
I agree with your words about the security and antivirus it's the same example idd. you are not 100% save with ACE as i have stated above that's why you need your own expert admin team who look after your server while you are away just as we do.
Also let me clear that you always need a solid valid proof, what you have said is just a theory as i proved it wrong points already. you always need to contact the mod developers if you want to understand it correctly.
Also note that using a different anticheats in the same time can cause your server to lag and fully screw it up sometimes with issues such as the timeout issues... because ACE is heavy enough plus the new cheats are are usually made to bypass all of the old anticheats so using them is pretty useless anyway.
You won't find programmers this days who are skilled enough to program anticheat, because anticheat programming is totally different from the mods/mutators programming. you need someone real real skilled enough such as Anth to do it.
I'm already in contact with Anth (ACE author) as i have heard he will continue the anticheat programming, he is going to do something within the next 3 month as he said i hope.
You don't need to worry about the cheaters in our server they usually get banned faster than what they can expect, we have our own skilled admin team and our private tools which help in recognizing who cheats and who doesn't.
However If you are still suspecting in someone you can record a demo and send it to us to check it out.
Note that: sniper and shockrifle is the most popular weapons by the normal weapon newbies because they are simply a long range weapon who are skilled with those weapons doesn't have to be a cheater because 90% of nw players this days just use this 2 weapons and skilled with them as well.
Some normal weapon players just use shock and sniper because that's how they are practiced to play normal weapons, but i still call them newbies because the point from nw is using all the weapons not just 2 weapons...
Oh well, about fabs there is really many fabs with different IP - ID however it's true that it could be one of them just sticking in your server so you gotta watch out when you ban a person with a common name such as fabs. make sure to rename the ban after you does ban him (in case if you think he is cheating for example) so the other innocent fabs won't get caught in the ban the same thing apply for the players who uses nick: player4
If you want to keep UT clean you gotta contact Anth he is the only person who supports UT against the cheaters and he deserve to be supported.
anyway in my opinion based on my experience the "[NSC-SYS] Login denied. Reason: Illegal login parameters" is just a timeout issue not more or less. so it got nothing to do with hacks or hackers.
I hope my posts have helped you to understand the case. if not, i recommend you to contact:
1. Anth the ACE Author.
2. Defrost the nexgen author.
those are the best people to explain the things better than anyone else about their own mods, also they are trusted.
At the end everyone have his own believes which he believes with, we can always improve our believes by learning more about the things by asking the right people for explanation and help such as Anth - Defrost etc...