"Whoever it was/is" is the subject of "must be very angry". But, "whoever" is the object of it was/it is and therefore should be "whomever". Both sentences should start with "whomever". Here is an example of using "whoever" in a very similar sentence: Whoever vandalized the classroom must be very angry.

I see nothing wrong with these, though I feel these are more commonly spoken instead of written. Subject: it Verb: be (must be)

Yes, they are both correct.