A: Yes. If someone is using the power of The Crown to pass on their turn, or the power of the MacGuffin or Backup MacGuffin to pick it up and put it back down as a turn option, then using Don’t Do That will result in the discard of the Item in question. It would also cause Plan C to be discarded rather than put back in the owner’s hand.

However, the passive effect of having people call you Your Majesty does not constitute a “use” which could be stopped with Don’t Do That, so The Crown can only be discarded with Don’t Do That if the player is using the passing power.