Using Planeswalkers as a distraction... That's actually won me a few games..
As for trample damage, you wouldn't need to
redirect it even if you could. You just throw the trampler at the walker and do damage with the excess. Not sure if that was clarified above, so I just wanted to point that out. The damage is already assigned to the walker.
And I don't think axe/shadows combo would work as the wording on
Spiteful Shadows specifies that the
redirect is aimed at the creature's controller. A Planeswalker card is not controlling that creature.