Disabling BlackUser would / could help... and I've recently had to do just that on an over damage issue.
You are right, high XP is anti-hack... and the style of disconnect changes, but it's still basically the server saying "I'm not going to talk to this client coz he isn't playing fair, so it's no fun."
I wasn't aware there are DC4 patched exes
Impaled. Thanks for that, it would be interesting to see how that is done some time.
Philosophically I differ from the trend here. I don't like players levelling too fast. It took me a year to get my first character to level 15, and the level cap was 29 when I started playing, that was really fun... reaching level 80 in a week, just isn't... sorry.
But when PT went Pay2Play, they made levelling to 40 really easy, as that was when you start paying... I was then 35. That lead to an exponential trend in the levelling which I'm not keen on either. If the progression was more even, so that it's not silly easy at low level and silly hard at high level, that would be a very good thing IMHO...
But without being able to change the level:xp ratio tables successfully, you would be better doing the opposite, and having lower XP multiplier on the server hotuk, and higher XP in the monster files.
Your goals are different to mine (I think) so this may not be a good answer for you. But I do think that what you are doing is the right thing to achieve what you want, and you only need to keep tweaking till it stops breaking, and settle for that as the best you can do.
I see it like overclocking your CPU. Some CPUs overclock better than others, some people are better / luckier at overclocking than others. Regardless, it's always a balancing act, and it's all great and good fun until it breaks.

Best of luck to you.

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This post doesn't really answer any of your questions, I know, but I send it to encourage you to keep going as you are. I don't think there is
one right answer for what you are trying to do, but you seem possessed of all the details to succeed in the end. I think it's just a matter of trial and error. If anyone knows differently, as Impaled proved some do, please share. ^_^