Can I summon Interplanetarypurplythorny Dragon using Witch of the Black Forest's effect
You will not be able to get the Interplanetarypurplythorny Dragon from the effect of Witch of the Black Forest's and activate its effect.

This is because the last thing to happen isn't a monster being destroyed but rather a monster being added to hand.

If you already had Interplanetarypurplythorny Dragon in your hand and Witch of the Black Forest is being destroyed, you still retain the priority to activate an effect after the mandatory effect of Witch of the Black Forest goes on the chain.