I'm not sure that using "I" is "formal" language necessarily.

Not necessarily. All examples you gave, teri, are approved language in all levels. The usage "It was me" as a whole sentence frequently occurs in colloquial language, exactly meaning "It was I" (... who committed the offence). Or "It was him" meaning (in formal language) "It was he", etc. Where is the difficulty?

The interrogative pronoun tends to the other direction in colloquial language: "Who did they arrest?" for (formal usage) "Whom did they arrest?"

Each dialect, pidgin language, etc. can have different grammar rules. Is the formal language awkward? Not at all, just those who want to use it should try to learn it properly, otherwise they do have a problem. (I am excused, being a furriner.)