The correct answers are The actor whom we hired is late again. and This is the script that I wrote last year.

When you use a relative pronoun ("that", "which", "whom") to connect clauses, it acts as the object of the relative clause. Therefore, you must drop the original object pronoun ("it", "him", "them").

Saying "the explosion that we filmed it" is like saying the object twice! English prefers you to keep it simple: just "the explosion that we filmed."