Under all of those possibilities, in correct, "formal" English (English almost never spoken, but grammatically correct), it should read:
"It could have been I."
It would also be "I" instead of "me" for every one of your examples and, in the answer above, it should read:
"It could have been he (not him) who died that day."
This is an example of the "predicate nominative" in English. Most English speakers say things such as "It's me" or "It's him or us", but correct English (formal English used especially in writing) would say "It is I" or "It is he or we". For instance:
"It is I who am to blame."
"It is we who have to live on this planet so it is we who must protect it."
"It is they who are the reason that we have failed."
"It is she whom you want."
Again, this is very formal English; therefore, I am teaching you the English that you would use when you are writing a formal essay, paper, dissertation, treatise, etc. in English. This would almost never be heard in Modern English conversation, however, wherein the "disjunctive pronoun", which is in the oblique case, is always used in colloquial speech. Sigh, it is not I who write the rules. (Notice the subject-verb agreement between "I" and "write".)