A: No, the Surprise you draw via Mystery Play* cannot be used to stop the previous Canceling of your play. It’s far too late. Whatever card you played has been Canceled, and that’s that. With no Surprise available, you could not stop the Surprise, so you have moved on and taken your next game action: choosing to use Mystery Play.

Since choosing to use Mystery Play will always disrupt the immediacy required for a reactive Surprise play, any Surprise drawn via Mystery Play would have to be used for it’s in-turn function.

* Or any of its analogues below:
Mystery Play in Fluxx 5.0, SE, Remixx, Astronomy, SpongeBob, and Wonderland
Mythtery Play in Fantasy
Wormhole in Star, Star Trek TOS, TNG, and Voyager
Shiny! in Firefly
Allons-y/Geronimo! in Doctor Who
Spontaneous Reaction in Chemistry
Egads! in Batman
Unknown Variable in Math
THWIP! in Marvel
(the Infinity Gauntlet Keeper in Marvel has this as its special power as well)
Great Idea! in Stoner
Magic Spell in Fairy Tale
Magic Portal in Adventure Time
Magic Shoes in Oz (if you click your heels together three times)
Open The Door in Monster (if the Spooky Door is in play)
Open A Gift! in Holiday (if The Gift is in play)
Chemical X in Cartoon Network (if at least one Powerpuff Girl is in play)
Time Doorway in Regular Show (if the Time Machine is in play)