A: To review, Play All Plus 1 says: Play all of the cards in your hand. Then take 1 card from the top of the draw pile and play that as well. This immediately ends your turn.

You do not have to take that last +1 play as soon as you play the last card from your hand. You can still take optional Free Actions granted by Rules, or use special powers granted by Keepers or Creepers. Taking that final +1 draw-and-play-it will be your final play, however, as that’s what invokes the “your turn then ends immediately” portion of the instructions.

Imagine that Play All Plus 1 is like adding a mandatory Free Action Rule to the table that says “At any time you have played all the cards in your hand down to zero, you may Draw 1 and Play It* and your turn ends immediately”. It’s like other Free Action Rules, in that you can do other Free Actions before it, but it’s also part of the Play Rule itself, and therefore mandatory, so if you’ve run out of other things to do (i.e. other Free Actions) you’ve got to do that last one and end your turn.

Also see: Order of events in a Fluxx turn

*i.e. Mystery Play or any of its analogues.