just had a few question before I attempt to Kill a Wither and probably inevitably fail...

Is Mob Griefing turned off? Like, could I spawn a Wither in a sort of cage and kill it, or will just burst out and massacre me?

Also, Dragon's Breath? As far as I know, this can only be obtained by going to The End and scooping up some Magic breath from the Dragon. Assuming the Dragons already been killed, how do I obtain this potion??

I'm assuming this is in regards to DMU based on certain wording, so:
I believe yes, mob griefing I think is turned off

For the dragon, you could technically respawn it maybe? I don't know the rules for that on DMU. Better option is to find someone selling it though, you'll need something to trade for it though