What happens if a creature is returned to owner's hand after blocking or being targetted?
Example 1:

If a spell with targets would resolve, and if it has no targets left, than that spell will be countered.

This is known as fizzle-ing

*note if the spell still has at least 1 target, it will still resolve.  chandra&#39;s outrage only targets a creature, and therefore will be countered, but flames of the firebrand will not be, if it still has at least 1 target left.

Example 1':

no, player B would not be able to target the wall if it dies.  It will be put into the graveyard as a state-based effect before anyone gets priority to cast spells or activate abilities.

Example 2:

You can activate  crystal shard after blockers have been declared and before combat damage is dealt.  the 2/2 will return to your hand, and the 3/3 will still be considered blocked

Once combat damage is dealt it's too late.  the 2/2 will go to the graveyard as a state-based effect before anyone gets priority to cast spells or activate abilities.