Q:Can Planeswalkers be the target of spells?
Yes, if it would say “target player”:
306.7. If noncombat damage would be dealt to a player by a source controlled by an opponent, that opponent may have that source deal that damage to a planeswalker the first player controls instead. This is a redirection effect (see rule 614.9) and is subject to the normal rules for ordering replacement effects (see rule 616). The opponent chooses whether to redirect the damage as the redirection effect is applied.
Keep in mind, if your opponent cannot be targeted by such spells, then you can’t have the damage redirected from them to their planeswalker.