QAn opponent only has an engaged Bellringer Angel on the field, and I attack it with a Maisha, Hero of Purgation. If I play a Purgation's Blade with Maisha, Hero of Purgation's Strike ability to destroy the enemy Bellringer Angel, can I change the target of Maisha, Hero of Purgation's attack to their leader?

ANo. Unless otherwise stated, you can't change the target of an attack, even if the original target leaves the field.

QIf I can't select an enemy follower on the field, can I still play a Purgation's Blade to give each Maisha, Hero of Purgation on my field attack equal to the number of followers in my cemetery?

ANo. If you can't select an enemy follower on the field, you can't play a Purgation's Blade.

QIf I have 4 or less cards in my deck, can I still give this follower Storm with its On Evolve ability?

AYes. Whenever you are required to bury more cards than you have, you will bury as many cards as possible before resolving the rest of the effect. You also can't lose the game as a result of resolving a bury effect.