I like to know about abuse on npc quest manager first.
What benefit do player get from abusing it?
The only thing I can think of is timing the quest. The quests rotates every 10 seconds, so you'll always get the quests you want if you have stopwatch beside you.
If that was the abuse, it's just inefficiency from the system.
Beside, it's not suppose to be about how long the player play in a day, but about how to keep them playing for longer term in time like years
This will be like you win the battle, but you lose the war.
.

I feels like this has some connection with 200k fame for event and dummies.
Before it's 200k, but you raised to 250k, now it's back to 200k.
So now you monitor it from this npc.
And if you really want to reduce dummies, you need to think along the line "how to stop getting new players"
Those people are try-hard with extra effort to spare. Making a high standard to not let them have it, will make the other player suffer from it.
Basically we're screwed either way.
It's an irony that it's a mmo, but multiplayer is the problem.