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You don't really have to mess up with client, just edit UserInfo/CharInfo packets, there should be something related to cursed weapons, if you want to disable it entirely, change whatever code it has to 0x00.

A config wouldn't change anything, you just need to move your announcement after "Welcome to Lineage II" message.
Look in EnterWorld.java for:
// Close lock at login activeChar.setLocked(false); and place your code before that. Also remove static from everywhere you added it and change
 
Olympiad.olympiadEnd(activeChar);
to
Olympiad.getInstance().olympiadEnd(activeChar);

What kind of help are you looking for?

Go to L2AttackableAI.java and search for Config.GUARD_ATTACK_AGGRO_MOB.
 
Paste here the part of the code surrounding Config.GUARD_ATTACK_AGGRO_MOB, so we can take a better look.

Here you go:
ThreadPoolManager.getInstance().scheduleGeneral(new Task(), 7200000); You can change the time here, in milliseconds.

There shouldn't be an issue there, obviously the original authors of this pack knew about this and added the catch (StackOverflowError). The only way to resolve this, is by checking all the calls of this method (infinite recursive calls are most likely the issue to this problem).